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Capping Ligand Effects on the Amorphous-to-Crystalline Transition of CdSe Nanoparticles
Abstract
Amorphous CdSe nanoparticles were prepared by a base-catalyzed room-temperature reaction between cadmium nitrate and selenourea, with dodecanethiol as a capping ligand. The nanoparticle size could be controlled from 1.9 to 3.6 nm by increasing the water concentration in the reaction. When the nanoparticles were heated in a pyridine suspension, excitonic peaks appeared in the initially featureless optical absorption spectra. By changing the suspension solvent and the capping ligand and its concentration, it was shown that the dynamic surface exchange between the ligand and pyridine controls the crystallization process. This phenomenon was interpreted as a surface rigidity effect imposed by the ligand, whose importance was separately evidenced on the dried nanoparticles by the evolution of X-ray diffraction patterns and Raman spectra. In particular, both techniques showed that a threshold temperature is needed before crystallization occurs, and such a threshold was related to ligand desorption. The surface effect was directly visualized by high-resolution transmission electron microscopy observations of the amorphous particles, where crystallization under the electron beam was observed to start by the formation of a crystalline nucleus in the nanoparticle interior and then to extend to the whole structure.
... NP can be produced either in amorphous or crystalline states. The crystallization process which play important role in modern science and technology [1,2] has been investigated by both experiment and simulation [4][5][6]. Computational simulations have been successfully conducted to study the amorphous solid-crystal transitions at atomic levels because simulation has advance on probing those transitions since it allows to calculating the trajectory of individual atoms [7][8][9]. ...
... Furthermore, it was revealed multiple intermediate states between disordered and crystalline phases. For Cu-Ni system [17,18] Although the nanoparticle (NP) has been intensively investigated by simulation and experiment [4][5][6]19], the crystallization as well as the microstructure of Fe NP remain poorly understood. Namely, the structure heterogeneity and effect of size for NP is still unclear yet. ...
We use the molecular dynamics simulation to study iron nanoparticles (NPs) consisting of 4000, 5000, 6000 atoms at temperatures of 300 and 900 K. The crystallization and microstructure were analyzed through the pair radial distribution function (PRDF), the potential energy per atom, the distribution of atom types and dynamical local structure parameters , where x is the bcc, ico or 14. The simulation indicated that amorphous NP contains a large number of ico-type atoms that play a role in preventing the crystallization. Amorphous NP is crystallized through transformations of f14 > 0 and fbcc = 0 type to bcc-type atoms when it is annealed at 900 K upon 40 ns. The growth of crystal clusters happens parallel with changing its microstructure. The behavior of the crystal cluster resembles the nucleation process described by classical nucleation theory. Furthermore, we found that the amorphous NP has two parts: the core has the structure similar to the one of amorphous bulk, in while the surface structure is more porous amorphous. Unlike amorphous NP, the crystalline NP also has three parts: the core is the bcc, the next part the distorted bcc and the surface is amorphous. Amorphous and crystalline NPs have part core which has the structure not depend on size.
Doi: 10.28991/HIJ-2021-02-03-01
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... This has hardly been addressed up to now. [50][51][52] The crystallization of initially disordered 2 nm triphenylphosphine-capped CdSe particles was observed in a 200°C hot solution and the process was attributed to the gradual healing of surface defects enabling relaxation reactions and surface reconstruction. 51 For disordered dodecanethiol-stabilized CdSe nanoparticles dynamic surface-exchange phenomena of coordinating solvent and ligand molecules were used to explain the observed post-synthesis crystallization kinetics in heated pyridine solutions. ...
... 51 For disordered dodecanethiol-stabilized CdSe nanoparticles dynamic surface-exchange phenomena of coordinating solvent and ligand molecules were used to explain the observed post-synthesis crystallization kinetics in heated pyridine solutions. 50 Within this study, we observed different crystallization rates induced by different organic ligands. To our knowledge, this is the first time that the influence of organic ligands onto the internal crystallinity of nucleating particles has been monitored directly. ...
Recent total scattering experiments have opened up the possibility to study nanoparticle formation
in-situ and to observe the structural transformation from precursor clusters to adult particles. Organic ligand molecules interact with precursors of metal oxide nanoparticles, yet their influence onto the evolution of crystallinity during particle formation has not been addressed in detail; nor have in-situtotal scattering experiments ventured into the field of low-concentration, room-temperature syntheses in organic solvents to date. In this report,we follow the crystallization of
ZnO nanoparticles in ethanol in the presence of different organic ligands. Low coordinated zinc precursor clusters rapidly polymerize upon base addition to particles of ca. 1 nm in diameter. In
-situ SAXS experiments reveal that the overall particle size increases to 2 to 4 nm with advancing reaction time. Complementary in-situ PDF experiments show smaller crystalline domain sizes, which are only one third to half as large as the particle diameter. The ZnO particles thus feature a crystalline core surrounded by a disordered shell. [...]
... Microemulsions are well known for the fabrication of small sized nanoparticles at low temperature and short processing time 21 while the capping agents can be used to achieve the desired crystallographic growth. 22 27 However, these studies showed that CdTiO 3 alone has no significant photocatalytic activity. Chloride is well known to strongly influence the crystallinity of cadmium chalcogenides. ...
... 37 It is believed that when a ligand is adsorbed on the surface of a growing nanoparticle, the surface acts as a rigid barrier against the atomic rearrangement and ligands are desorbed during heating which favors the particle crystallization. 22 Therefore, we suggest that during our experiments, chloride gets adsorbed on the surface of growing CdTiO 3 . The adsorbed chloride gets desorbed during calcination due to which CdTiO 3 with increased crystallinity is obtained. ...
The crystallinity of cadmium titanate (CdTiO3) was greatly improved when synthesized under mild reaction conditions, in the presence of chloride. The highly crystalline CdTiO3 showed much enhanced photodegradation of methyl orange (MO) under simulated sunlight. CdTiO3 was characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N2 adsorption/desorption, photoluminescence (PL), and UV/vis spectrometry. The enhanced photodegradation was attributed to the better charge separation owing to its higher crystallinity.
... A shoulder near 170 cm −1 can be seen, which is attributed to the transverse optical (TO) phonon mode of CdSe nanoparticles. 52 A small and broad peak at about 264 cm −1 is also apparent in the spectrum. ...
... And these SO modes originate due to the presence of more number of atoms on the surface as compared to the bulk in very small sized nanoclusters, behaving like molecules. 53 A similar band was observed by Epifani et al. 52 in the case of CdSe nanoparticles at around 270 cm −1 and was assigned to the SO modes. Furthermore, the presence of amorphous-Se has also been reported to exhibit a Raman peak at around 264 cm −1 . ...
RTILs as media to synthesize a variety of nanomaterials are gaining momentum owing to their unique physicochemical properties. However, the fundamental questions regarding the role of the inherent structure of the IL in directing the morphology and the growth mechanism of the nanoparticles are still unexplored. Therefore, an attempt was made in this respect wherein CdSe nanoparticles were synthesized in a neat room temperature ionic liquid (RTIL), 1-ethyl-3-methyl imidazolium ethylsulfate ([EMIM][EtSO4]), under ambient conditions. The IL was found to play three roles, as a solvent, as a stabilizing agent and as a shape directing template. The primary nanoparticles were of the sizes in the range of 2-5 nm, as determined by HR-TEM. These primary nanoparticles grow into nanoflake-like units which further self-assemble and transform into a mixture of anisotropic nanostructures (predominantly 2D sheets and flower-like 3D patterns) as revealed by the SEM studies. The co-existence as well as the stability of these nanomorphologies point towards the intrinsic microheterogeneity prevailing in the IL. Furthermore, the vibrational spectroscopic studies comprising of FT-IR and Raman spectroscopy clearly indicate a sort of accord involving the π-π stacked aromatic geometry and the hydrogen bonding network (between the cation and the anion) of the IL with the CdSe nanoparticles. Therefore, a suitable mechanism has been provided for the resulting anisotropic nanostructures on the basis of the structural and the fluidic aspects of the IL in conjunction with the surface properties of the transient morphologies involved in the process. To further supplement this, control experiments were facilitated by diluting the IL with different amounts of water and the morphology of the CdSe nanostructures was examined at respective mole fractions of water as well as at different time intervals.
... It has been shown that substantial control of the synthesis and properties of NCs can be achieved by selecting the appropriate ligands in synthesis processes. The organic capping ligands used in the colloidal synthesis have a profound impact on the nanocrystals' shape, size, composition and morphology [25][26][27][28][29][30][31][32]. A crucial step to design biocompatible quantum dots for biological application is to choose appropriate surface ligands with high affinity and selectivity that can target directly to the binding site of the protein of interest [33,34]. ...
... It has been reported very recently that surface ligands can control the synthesis of magic-size quantum dots [68][69][70]. Paralleling these fields, peculiar structure transformation can be induced by surface modifications [26], as evidenced on the equilibrium between the wurtzite and zinc blende polytypes of CdSe nanocrystals, where it has been shown that short-chain phosphonic acids stabilize the zinc blende phase whereas octadecylphosphonic acids stabilize the wurtzite phase [71]. ...
Surface effects significantly influence the functionality of semiconductor nanocrystals. High quality nanocrystals can be
achieved with good control of surface passivation by various hydrophobic ligands. In this work, the chemistry between CdSe
quantum dots and common surface capping ligands is investigated using density functional theory (DFT). We discuss the electronic
structures and optical properties of small CdSe clusters controlled by their size of particle, self-organization, capping
ligands, and positive charges. The chosen model ligands reproduce good structural and energetic description of the interactions
between the ligands and quantum dots. In order to capture the chemical nature and energetics of the interactions between the
capping ligands and CdSe quantum dots, we found that PMe3 is needed to adequately model trioctylphosphine (TOP), NH3 is sufficient for amines, while OPH2Me could be used to model trioctylphosphine oxide. The relative binding interaction strength between ligands was found to
decrease in order Cd–O>Cd–N>Cd–P with average binding energy per ligand being −25kcal/mol for OPH2Me, −20kcal/mol for NH3 and −10kcal/mol for PMe3. Charges on studied stoichiometric clusters were found to have a significant effect on their structures, binding energies,
and optical properties.
KeywordsCdSe quantum dots–Surface ligands–Charges–Density functional theory
... Further, surfactants can act as perturbants to cause phase transformation at the postsynthetic process. Examples include alteration in the ZnS crystallinity via interaction with water, 23 amorphous to crystallization in CdSe by the capping agent, 24 reversible switching of the crystallinity in Pd nanoparticle via exchanging the surface capping agent, 25 and hcp (2H/4H) to fcc irreversible phase transformation in Au nanostructures 11,26 by exchanging surface capping agents. In all cases, the switching in crystallinity is mainly governed by the binding strength of the adsorbents with the nanoparticle surface scarcely changing the morphology. ...
... To study the crystallization process of metals has been a successful studied by experimental methods and simulated methods [30][31][32][33][34][35]. The results showed that there is a transition from the crystal state to the amorphous state [36][37][38][39][40][41]. ...
The content of this article, study crystallization process of Al metal by Molecular Dynamics simulation method with embedded interaction potential Sutton-Chen (SC) and recirculating boundary conditions. The results of the study influence of the heating rate 4x1014 K/s, 2x1014 K/s, 4x1013 K/s, 2x1013 K/s, 4x1012 K/s with Mal8788; Al metal with atoms number (N), N = 4000 atoms (Mal4000), 5324 atoms (Mal5324), 6912 atoms (Mal6912), 8788 atoms (Mal8788) at temperature (T), T = 300 K; annealing time (t), t = 0 ps, 50 ps, 100 ps, 150 ps, 250 ps of Mal8788 at the glass temperature (Tg), Tg = 600 K. The results got the show, when increases heating rate leads to the crystallization process of Al metal decreases correspond to the process move from crystallization state to an amorphous state and heating rate appropriately is 4x1012 K/s. When increases atomic number (N) at the temperature (T), T = 300 K, and the increases tempering time (t) at glass temperature (Tg), Tg = 600 K then the structures units number Face-Centred Cubic (FCC), Hexagonal Close-Packed (HCP) increases, Amorphous (Amor) decreases correspond to r = 2.80 Å, size (l) increases, the total energy of the system (Etot) decreases, these results satisfies with l ~ N-1/3, Etot ~ N-1. The results got a show, at temperature T = 300 K, the metal Mal8788 always at amorphous and at T = 800 K it is always liquid. When increasing time annealing does not change the structure and crystallization process. When increasing tempering time at glass temperature (Tg), Tg = 600 K leads to the crystallization process increased corresponding to structure units number FCC, HCP increases, Amor decreases
... Nghĩa là, hạt nano Fe VĐH đã tinh thể hoàn toàn trong giai đoạn ba. Phân tích trên cho thấy, quá trình tinh thể hạt nano Fe VĐH thành cấu trúc bcc rất phù hợp tốt với lý thuyết nhân tinh thể cổ điển [10]. Để hiểu rõ hơn về quá trình hình thành và phát triển của các đám tinh thể trong hạt nano Fe VĐH ở 900 K, chúng tôi chia hạt nano này thành 5 vùng khác nhau như được minh họa trên hình bên trong của hình 4. Ở đây, vùng 1 là một quả cầu có bán kính 5,4 Å. Các vùng còn lại là các lớp cầu có độ dày liên tiếp bằng 5,4 Å. Như thấy trên hình 4, sau thời gian 8 × 10 6 bước mô phỏng đầu tiên, có một vài nhân với kích thước nhỏ hơn 50 nguyên tử được tạo thành trong tất cả các vùng thể tích của hạt nano và biến mất theo thời gian hồi phục. ...
This paper studies the crystallization process and structure of amorphous iron nanoparticles by molecular dynamics method. The study shows that amorphous iron nanoparticles could not be crystallized at 300 K and 500 K. Iron nanoparticle, annealed at 900 K over a long time, was crystallized into a BCC crystal structure. The structure of crystallized iron nanoparticle at 900 K was analyzed through the pair radial distribution function and the number of crystal atoms upon various regions in nanoparticles. The simulation revealed that the first nuclei was formed most frequently in the area near the surface of the nanoparticle. Then the crystal cluster grew toward the centre of the nanoparticle. The completely crystallized nanoparticle had two components: the core with a BCC crystal structure and surface with an amorphous structure. As for the amorphous nanoparticle at 300 or 500 K, crystal-clusters were too small to grow large enough to crystallize the nanoparticle. Keywords Iron nanoparticle, crystallize, annealing, crystal atom, crystal cluster. References [1] J.D. Honeycutt, C.H. Andersen, Molecular dynamics study of melting and freezing of small Lennard-Jones clusters, Journal of Physical Chemistry 91 (1987) 4950-4963. https://doi.org/ 10.1021/j100303a014.[2] H. Shin, H.S. Jung, K.S. Hong and J.K. Lee, Crystallization process of TiO2 nanoparticles in an acidic solution, Chemistry letters 33 (2004) 1382-1383. https://doi.org/10.1246/cl.2004. 1382.[3] D. Shi, Z. Li, Y. Zhang, X. Kou, L. Wang, J. Wang, J. Li, Synthesis and characterizations of amorphous titania nanoparticles, Nanoscience and Nanotechnology Letters 1 (2009) 165-170. https://doi.org/10.1166/nnl.2009.1037.[4] D.N. Srivastava, N. Perkas, A. Gedanken, I. Felner, Sonochemical synthesis of mesoporous iron oxide and accounts of its magnetic and catalytic properties, The Journal of Physical Chemistry B 106 (2002) 1878-1883. https://doi. org/10.1021/jp015532w.[5] N. Zaim, A. Zaim and M. Kerouad, The hysteresis behavior of an amorphous core/shell magnetic nanoparticle, Physica B: Condensed Matter 549 (2018) 102-106. https://doi.org/ 10.1016/j.physb. 2017.10.071.[6] L. Gao and Q. Zhang, Effects of amorphous contents and particle size on the photocatalytic properties of TiO2 nanoparticles, Scripta materialia 44 (2001) 1195-1198. https://doi.org/ 10. 1016/S1359-6462(01)00681-9.[7] G. Madras, B.J. McCoy, Kinetic model for transformation from nanosized amorphous TiO2 to anatase, Crystal growth & design 7 (2007) 250-253. https://doi.org/10.1021/cg060272z.[8] C.I. Wu, J.W. Huang, Y.L. Wen, S.B. Wen, Y.H. Shen, M.Y. Yeh, Preparation of TiO2 nanoparticles by supercritical carbon dioxide, Materials Letters 62 (2008) 1923-1926. https://doi.org/10. 1016/j.matlet.2007.10.043.[9] C. Pan, P. Shen and S.Y. Chen, Condensation and crystallization and coalescence of amorphous Al2O3 nanoparticles, Journal of crystal growth 299 (2007) 393-398. https://doi.org/ 10. 1016/j.jcrysgro.2006.12.006.[10] M. Epifani, E. Pellicer, J. Arbiol, N. Sergent, T. Pagnier, J.R. Morante, Capping ligand effects on the amorphous-to-crystalline transition of CdSe nanoparticles, Langmuir 24 (2008) 11182-11188. https://doi.org/10.1021/la801859z.[11] P.H. Kien, M.T. Lan, N.T. Dung, P.K. Hung, Annealing study of amorphous bulk and nanoparticle iron using molecular dynamics simulation. International Journal of Modern Physics B 28 (2014) 1450155 (17 page). https:// doi.org/10.1142/S0217979214501550.[12] V.V. Hoang and N.H. Cuong, Local icosahedral order and thermodynamics of simulated amorphous Fe. Physica B: Condensed Matter 404 (2009) 340-346. https://doi.org/10.1016/ j.physb. 2008.10.057.
... The stability of amorphous NPs against crystallization plays an important role because of this related to their working ability in practice. The crystallization of amorphous NPs is studied intensively by experiments [20][21][22][23][24][25][26]. It was shown that the crystallization in NP proceeds via the nucleation, but exhibits certain specific features comparing to the bulk counterpart. ...
The FeB nanoparticle consisting of 5000 particles (4500 Fe atoms and 500 B atoms) have been investigated by means of molecular dynamics (MD) simulation. When the amorphous FeB nanoparticle is annealed at temperature of 900 K for a long time, it is crystallized into bcc crystalline structure. The simulation shows that the sample undergoes crystallization via the nucleation mechanism. During the crystallization, B atoms diffuse to the boundary region of Fe crystal. The crystal growth proceeds when this boundary region attains specific properties which are defined by the fraction of B atoms and the energies of AB-atoms and CB-atoms. Further our study indicates that the crystalline and mixed FeB nanoparticles consists of three distinct parts including Fe crystalline and two FeB amorphous parts (B-poor and B-rich amorphous part). The different polymorphs of FeB nanoparticle differs in the local structure, size of Fe crystal and energies of different type atoms.
... Goethite can form from ferrihydrite largely by relocation of a subset of iron atoms into adjacent facesharing octahedral sites and small displacements of every fourth oxygen plane (Banfield et al., 2000), and amorphous aluminum hydroxide could form similar structures during the crystallization process. The structural metastability induces the amorphous-tocrystalline transition of the nanoparticles under mild conditions (Epifani et al., 2008). ...
When aluminum salts are added to water at around neutral pH, a precipitate of Al hydroxide is formed very rapidly. Initially the precipitate is in the form of nano-scale primary particles, which then aggregate to form flocs. The nature of the flocs depends greatly on the solution composition, for instance on the presence of humic acid (HA), which not only increases the size of the primary nanoparticles, but also decreases the connection points between them. The nanoparticles become smaller with aging, both with and without HA, as a result of crystallization. The aggregated amorphous nanoparticles (settled flocs) undergo a room temperature structural modification best characterized as a disorder-to-order transition, following elimination of water. During this process, the apparent Al concentration in the supernatant of water increases with age. The "dissolved Al" concentration in the supernatant becomes higher with increasing pH and, to some extent, in the presence of HA. However, it can be shown that the "dissolved Al" in the supernatant exists in the form of crystalline nano-particles or larger clusters, which are detached from the settled flocs. TEM results confirmed that HA only adsorbed on the surface of nano-particles during the coagulation process, which shows precipitate nanoparticles formed firstly during sweep coagulation before the adsorption of HA or complexed Al3+-HA. However, the adsorbed outer layer of HA does not change the crystallization process for the inner part of nano-particles. This laboratory study may have implications for the release of Al from sediments into lake water, following addition of coagulants to lower phosphorus concentrations.
... These QDs have also been characterized by Raman as well as FTIR spectroscopy. One representative Raman spectra for the CdSe QDs synthesized from the 1:1 reaction mixture is shown in Fig. 5. Raman spectra of as grown CdSe QDs comprised of peaks for fundamental longitudinal optical (LO1) phonon mode at 202 cm À1 and the second longitudinal optical (LO2) phonon mode at 404 cm À1 [48,49]. In between these two peaks there are peaks at 242, 280, 321 and 350 cm À1 which may be surface optic modes [50] These types of surface optic modes have been reported earlier in very small size QDs, which arise due to more no. of atoms on the surface in a nanostructures behaving like molecules [50]. ...
CdSe quantum dots (QDs) have been synthesized in aqueous solutions of Cd(NH3)4SO4 and Na2SeSO3 in the presence of 1-thioglycerol at ambient conditions. These QDs were found to exhibit very sharp excitonic absorption peak between 400 and 430 nm and a second peak at around 360 nm and exhibit very broad fluorescence in the entire visible range with a large stokes shift. The QDs were monodisperse and stable under ambient conditions for several months. Their optical properties were found to have a strong correlation with the stoichiometric ratios of Cd:Se. 1-Thioglycerol was found to play a dual role in the synthesis of CdSe QDs, (i) catalyses the reaction to form CdSe and (ii) acts as a capping agent for these QDs. An in depth study of their electrochemical analysis have been made and a probable reaction mechanism for the formation of these QDs has been provided.
... [249] It is believed that when a ligand is adsorbed on the surface of a growing nanoparticle, the surface acts as a rigid barrier against the atomic rearrangement and ligands are desorbed during heating which favors the particle crystallization. [250] Therefore, we suggest that during our experiments, chloride gets adsorbed on the surface of growing CdTiO 3 . The adsorbed chloride gets desorbed during calcination due to which CdTiO 3 with increased crystallinity is obtained. ...
Titanium dioxide (TiO2), strontium titanate (SrTiO3), and cadmium titanate (CdTiO3) are three of the important titanium based oxides. They are semiconductors with wide band gap so they can absorb only UV light which constitutes about 5% of the solar radiations. These semiconductors also suffer from a fast charge recombination during photocatalytic reactions, which greatly reduces their photocatalytic efficiency. We aim to develop strategies for visible light activation and suppression of charge recombination for enhanced photocatalysis over TiO2, SrTiO3, and CdTiO3.
Ag/AgCl/TiO2 was synthesized and then CuO was introduced into it via a reverse microemulsion. CuO/Ag/AgCl/TiO2 composite was characterized and evaluated for the visible light photocatalysis. Degradation of about 42% MO and 53% phenol was achieved over Ag/AgCl/TiO2 under our experimental conditions. The visible light photocatalytic performance of Ag/AgCl/TiO2 was greatly enhanced by introducing a small amount of CuO into Ag/AgCl/TiO2. The MO and phenol degradation was increased up to 93% and 71%, respectively, when CuO/Ag/AgCl/TiO2 was used as photocatalyst under the same experimental conditions. Copper oxide is suggested to act as sink for photogenerated electrons, which resulted in better charge separation and hence enhanced photocatalysis.
Ag/AgX (X=Cl, Br) was loaded onto ATiO3 (A=Cd, Sr) to fabricate a new series of plasmonic photocatalysts. Pristine ATiO3 did not show activity for visible light photodegradation of MO or RhB. Ag/AgBr/SrTiO3 was found to be the best photocatalyst resulting in 93% MO degradation in 15 min and 96% RhB degradation for 25 min under visible light illumination. The enhanced activity is attributed to Ag SPR, visible light absorption by AgBr, and a more negative conduction band position of AgBr.
Chloride as a capping agent during the microemulsion-based synthesis was found to significantly improve the crystallinity of CdTiO3 which was found to greatly enhance the photodegradation of MO under solar light. About 27% degradation of MO was achieved from CdTiO3 synthesized in the absence of chloride. In contrast, about 98% degradation of MO was achieved from CdTiO3 synthesized in the presence of optimum amount of chloride. Better charge separation due to the high crystallinity resulted in improved photocatalytic activity.
... The crystallization of amorphous NPs has been studied intensively by experiments. [20][21][22][23][24][25][26] It was shown that compared to the bulk counterpart, the crystallization in NP comprises of specific processes originated from the porous structure of the NP surface. Still the crystallization mechanism at atomic level remains not fully understood yet. ...
The separation of Fe crystal from amorphous nanoparticle (NP) has been studied using molecular dynamics simulation. The simulation shows that the NP is crystallized through three stages. In the first stage NP undergoes the relaxation which results in forming nucleation regions where the atomic arrangement is similar to the distorted crystalline lattice. During the first stage the nuclei are unstable and dissolve for short times. In the second stage the stable crystal clusters have been created and new nuclei are formed mainly in the boundary region of crystal cluster. The stable crystal clustergrows in the direction to cover the core and then spreads out to the surface of NP. For the third stage the crystal clustergrows slightly with times. Further study concerns the different morphologies of NP. We found that the crystalline NP comprises a Fe crystalline grain with defects and separate clusters of Am-atom. Comparing to the amorphous NP, the structure organization of crystalline NP is more complicated and cannot be described by the simple shell/core model.
... The bands are shifted towards short wavelengths with an increase in V [Er3+] , which should result from an decrease in GS produced by the gradual incorporation of Er 3+ ion in the network. It may be due to incorporation of Er 3+ ion that a decrease in defects is produced, as noticed in the shift from the E g of bulk CdSe (∼712 nm), as a consequence of quantum confinement [19] and the position of a band is related to GS of the absorbing nanocrystals. Report an absorption maximum at ∼492-578 nm, a range very close to our results reported in this work [20]. ...
CdSe thin films were prepared by chemical bath and doped in situ with Er³⁺ ions. Three impurity levels were prepared by changing the relative volume of the salt solution containing Er³⁺ ions in the CdSe growing solution. Changes in the grain size (∼5.5-3.5 nm) and band gap energy (∼1.80-2.25 eV) were observed in the undoped and doped CdSe films, respectively. Photoluminescence studies displayed room temperature emission exhibiting NIR-to visible upconversion. The transition bands ⁴I13/2 → ⁴I15/2, ²I9/2 → ⁴I15/2 and ²F9/2 → ⁴F15/2 in the ∼700–850 nm region were investigated. Upconversion emissions were observed from the CdSeEr sample under light excitation (325 nm). The upconversion emission intensity ratio of these transitions is attributed to the variation of the local structure around Er³⁺ ions. These results confirm that visible upconversion emissions of Er³⁺ in the CdSeEr nanocrystals are mainly produced via two-photon excited-state absorption and energy transfer upconversion processes.
... The stability of amorphous nanoparticles ( ANP) against crystallization is of great interest, because this is related to their working ability in practice. The crystallization of ANP is studied mainly by experiments [18][19][20][21][22][23][24]. It was revealed that the glass and crystallization temperature of ANP is dependent on size. ...
... The XRD results shown in Fig. 3b indicate that there was little crystallization of newly formed sludge in the CUF-1 tank, but the clear presence of small peaks (Al(OH) 3 ) in the XRD spectra for the 7 day-aged sludge demonstrated that the sludge underwent an aging process involving crystallization. The observed amorphous-tocrystalline transition of the nanoparticles, under mild conditions, indicated that it is a surface-mediated phenomenon (Epifani et al., 2008). Therefore, it is concluded that the crystallization process probably caused the release of nano-scale particles from the sludge and into the membrane suspension in CUF-7. ...
This paper concerns a previously unreported mechanism of membrane ultrafiltration (UF) fouling when a UF process with coagulation pre-treatment is used in drinking water treatment. The significance of settled coagulant solids (sludge) with different age within the membrane tank on UF fouling has been investigated at laboratory-scale, using model micro-polluted surface water. The process of floc crystallization and increasing bacterial EPS with solids (sludge) retention time may be detrimental to UF operation by causing an increased rate of membrane fouling. In this study the performance of two alum pre-treated hollow-fibre UF units, operated in parallel but with different settled sludge retention times (1 and 7 days), was compared. The results showed that over 34 days of operation the extent of reversible and irreversible fouling was much greater for the 7-day solids retention time. This was attributed to the greater extent of bacterial activity and the presence of Al-nanoparticles, arising from sludge crystallization, at the longer retention time. In particular, greater quantities of organic matter, particularly EPS (proteins and polysaccharides), were found in the UF cake layer and pores for the 7-day retention time. The addition of chlorine later in the membrane run substantially reduced the rate of membrane fouling for both sludge retention times, and this corresponded to reduced quantities of organic substances, including EPS, in the cake layer and pores of both membranes. The results suggest that bacterial activity (and EPS production) is more important than the production of Al-nanoparticles from solids crystallization in causing membrane fouling. However, it is likely that both phenomena are interactive and possibly synergistic.
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... • Semiconductors: II-VI compounds (CdSe [23] [24], CdTe [25] [26], CdS [27], ZnSe, ZnTe, ZnS [28]), III-V compounds (GaAs [29], GaP, InAs [30], InP), IV-VI compounds (PbS, PbSe [31], PbTe [32]) and silicon [33]. ...
... The role of the capping molecules is crucial in nanoparticles synthesis: in fact they regulate their shape, size, chemical composition and morphology. [1][2][3][4] They affect their conductive properties simply acting as insulating layer or also they have deep influence on many their bulk-related physical properties, like relaxation of hot carriers inside the dot, 5 activation of non radiative decay channels, and/or charge trapping 6 possibly in steady state photoluminescence. Specific ligand's linker groups allow the control of quantum confinement of the exciton in the nanoparticles up to the exchange of photo-excited electron or holes between the nanoparticles and the ligand. ...
Multilayered films composed by CdSe Nanocrystals (NCs) interlinked by ethylene-1,2- bis(dithiocarbamate) or adipate anions were prepared on ITO glass via layer-by-layer alternation. The films were analyzed by UV-vis, TEM, Photoluminescence (PL) emission and Pump-Probe spectroscopy. While the PL emission of the two samples present no differences, femtosecond Pumpprobe experiments reveal an higher charge generation efficiency in bis(dithiocarbamate) based films than in dicarboxylate ones.
... However, the extremely broad peaks in the as-deposited composite thick films with CdSe composition greater than 23% are consistent with XRD patterns of both amorphous Se and amorphous CdSe. [39][40][41] The spectrum of the as-deposited pure CdSe thick film has peaks at 2q ¼ 25 , 41 , 45 , 49 , 52 , 63 , 71 which correspond respectively with the (002), (110), (103), (004), (112), (203), (105) crystal planes of hexagonal CdSe. No isolated peaks corresponding with the cubic phase of CdSe, Se, or Se x O y are seen in the thick film spectrum. ...
A unique fabrication method, oblique angle codeposition, is used to deposit well-aligned nanorod arrays and thick films of homogenously mixed CdSe–TiO2 composites. The composite films are characterized structurally, optically, and photoelectrochemically using a variety of experimental techniques. The CdSe–TiO2 composites are compared with pure CdSe and TiO2 films in order to determine their utility for photoelectrochemical (PEC) applications and to understand the mechanisms underlying the observed behaviors. The evaporation process of CdSe creates three different cluster types within the TiO2 film structures: isolated Se, Se-deficient CdSe, and Se-rich CdSe. The prevalence of each cluster type is dependent on predicted film composition, and each is affected differently by open-air annealing. Isolated Se can be incorporated into the TiO2 lattice, resulting in low energy rutile phase. Se-deficient CdSe clusters crystallize preferentially into cubic CdSe and are easily oxidized into CdO, while Se-rich CdSe clusters crystallize into hexagonal CdSe and are more stable. Furthermore, each of these cluster types interacts differently with the surrounding TiO2 matrix, resulting in diverse optical and PEC behaviors. Importantly, the composite nanorod structure is a more efficient photoanode under visible light illumination than both the pure CdSe and TiO2 nanorod array films. The stoichiometry of the CdSe domains is more important than overall CdSe content within the film in determining the structural, optical, and PEC properties of the films.
... In particular, when the very efficient hole-trapping ligand is applied, the role of the holes in the relaxation of electrons is diminished, and relaxation dynamics of solely electrons can be studied. 9 In addition, a pronounced effect of pyridine treatment onto the amorphous-to-crystalline transition 10 and self-assembling of small CdSe NCs into 1D nanowires 11 has been reported. The small size of pyridine molecules allows for close packing of NCs (with the sub-nm interparticle distance) and efficient electronic interaction between them. ...
The influence of ligand exchange for pyridine onto the structure and phonon spectra of oleic acid-stabilized CdSe nanocrystals (NCs) is studied by resonant Raman and optical absorption spectroscopy, nuclear magnetic resonance and transmission electron microscopy. The removal of oleic acid ligand by pyridine treatment results in change of intensity ratio of the longitudinal optical (LO) phonon peak to its overtones. The latter effect is attributed to a changed electron-phonon coupling in NCs upon introduction of the hole-capturing ligand (pyridine). The upward shift and broadening of the LO phonon peak are also observed and supposed to be the result of interplay between partial oxidation of the NC and strain induced by surface reconstruction. The relative contribution of these two effects is found to be dependent on the NC size. The activation of two additional Raman features, in the low-frequency range and above the LO band, for pyridine-treated NCs is supposed to be related with induced disorder or reconstruction on the NC surface. No noticeable effect of the surface treatment and concomitant NC aggregation onto the surface optical phonon mode was observed.
The contemporary lifestyle has embraced optics and photonics technologies in various ways spanning from communication, computation, energy sector, and medical science, to entertainment. Materials are playing a progressively important role in such technologies. In the last 20 years, low-dimensional materials such as quantum dots (QDs) and core–shell QDs (CSQD) comprising of compounds from II to VI group (CdSe, CdTe, etc.), have attracted attention owing to their cherished characteristics both for fundamental research and practical applications. They provide tunable emission through modifications in their sizes and shapes, and flourish in many areas such as photovoltaics, photo-electronics, light-emitting diode, nonlinear optics, and laser development. In this article, the prevalent synthetic methods are mentioned which employ rigorous conditions of temperature, pressure, passive atmosphere, toxic reducing agents, etc. Thereafter, a one-step efficient synthesis method is developed using an electron beam (EB). CdSe quantum dot (QD) is synthesized in homogeneous (aqueous) as well as in microheterogeneous systems (water-in-oil microemulsions, and ionic liquids) and characterized by recognized instrumental techniques. The radiolytic synthesis yields CdSe nanoparticles known to proceed through hydrated electrons (eaq−). Elegant pulse radiolysis technique is used to investigate the formation dynamics of these nanoparticles. Further, it is shown that the CSQD architecture has been effectively developed isolating the core QDs from the surrounding environment and in effect subdued the formation of defects/traps on the surface. By appropriately selecting first the materials for core and shell and then their size/shape and chemical composition, the band structure of these CSQDs can be modified. This overview focuses on the tailored optoelectronic properties and devices including solar cells, light-emitting diodes, optical switch/limiter, QD laser, and biological/chemical applications. These successes have maintained the momentum of this subject area for a wide-ranging group of scientists and technologists to work for future prospective applications.
Properties of nanomaterials such as optical, electrical, and chemical properties are strongly correlated with lattice symmetry, making characterization of lattice symmetry essential. We introduce a symmetry analysis method using 3D atomic coordinates obtained by Brownian one‐particle 3D reconstruction. The method allows direct and quantitative analysis of symmetrical properties and delivers local structural characteristics of individual platinum (Pt) nanoparticles in unit‐cell level. Local structural deformations of the Pt nanoparticles such as lattice distortion and internal symmetry breakage are demonstrated, revealing that the crystal structure of sub‐3 nm Pt nanoparticles generally maintains FCC crystallinity and exhibits localized deviation from their bulk counterpart.
Nanoparticles are usually obtained as organic-inorganic hybrids from wet-chemical syntheses, comprising of an inorganic core and a shell of organic capping ligands. The capping ligands are indispensable in the synthesis because they maintain the colloidal dispersity of nanoparticles and regulate the crystal growth into different shapes. It is difficult to obtain nanoparticles with designable shapes and surface properties directly from the synthesis. Ligand exchange opens a way to highly customizable surface properties of nanoparticles. As a result, the surface charge, functionality, accessibility of the surface sites by external molecules, dispersity and thus processability of the nanoparticles in different types of solvents, and charge transport properties across the nanoparticle assemblies, could be effectively tuned, which significantly broadens the application of the nanoparticles in different fields. This article summarizes the typical methods and mechanisms for the ligand exchange of nanoparticles that have been developed so far. In particular, ligand exchange strategies for phase transfer and clean surfaces of nanoparticles are extensively discussed. At the end, we provide a summary and some perspectives on the future development of the ligand exchange technology.
Thioglycolic acid (TGA) capped CdSe quantum dots (QD) have been synthesized in aqueous solution using facile, one‐pot and soft chemical route at ambient condition. The excitonic absorption peak and band gap of the QD could be tuned from 435 nm to 472 nm and from 2.37 eV to 2.60 eV respectively. The morphology of these QD were dependent on the precursor's concentration during synthesis. These QD exhibited broad photoluminescence (PL) spectra with large Stokes shift. It was observed that in the synthesis of CdSe QD TGA played twin role of catalyzing and capping agent. Detailed reaction mechanism of synthesis studied using cyclic voltammetry confirms the role of thioglycolate anion in the synthesis of CdSe QD. Cytotoxicity effects of these QD were studied in both the normal and cancer cell, indicted that TGA capped CdSe QD are cytotoxic beyond 10 μg/ml concentration and leads to cell proliferation of cancer cells.
We use the molecular dynamics simulation to study iron nanoparticles (NP) which consist of 5000 atoms at temperatures of 450 and 850 K. The crystallization and structure evolution was analyzed through pair radial distribution function, transition to different x-types, with x is the bcc, fcc and hcp, ico, 14, 12, and dynamical structure parameters. Simulation results that at 450 K, NP contains a large number of ico-type atoms which play a role in preventing of crystallization. The crystallization happened when NP was annealed at 850 K for 40 ns. Transitions to bcc-type do not happen arbitrarily at any location in NP, but instead they are focused in a non-equilibrium region. We showed that the crystallization pathway includes intermediate states between amorphous and crystalline phases. Firstly, a large cluster of cryst-atom is formed in a middle layer of NP. Next, this cluster grows up and the parameter for this cluster increases rapidly. Finally, the cluster of cryst-atom is located in a well-equilibrium region covered a major part of NP. The structure of crystalline NP is strongly heterogeneous and consists of separate local structure regions.
In this paper, crystallization pathway and dynamical heterogeneity (DH) in iron nanoparticle (NP) have been investigated in detail for spherical samples containing 5000 atoms, which were obtained by the molecular dynamics simulation based on Pak–Doyama potential. The crystallization was analyzed through pair radial distribution function, angle distribution, parameter 〈Fbcc〉 and transition to different x-types, where x is the bcc, fcc-hcp, ico, 14 or 12. We found that transitions to bcc-type do not happen arbitrarily at any location in NP, but instead they are concentrated in a nonequilibrium region. The crystallization pathway comprises of intermediate states between amorphous and crystalline ones. At the early stage, a large cluster of Cryst-atom formed is located in a middle layer of NP. Then, this cluster grows up and the parameter 〈Fbcc〉 for it increases rapidly. At the final stage, the cluster of Cryst-atom is located in a well-equilibrium region covering a major part of NP. It is found that the structure of amorphous and crystalline NPs is strongly heterogeneous and consists of separate regions with different local microstructure. This indicates the DH in NP. We also found that there is a connection between local structures and DH in NP.
In this paper, the nanoparticle (NP) Fe was investigated by means of molecular dynamics simulation. The crystallization mechanism was studied through the time evolution of crystal cluster and potential energies of different atom types. The simulation shows that the NP was crystallized into bcc crystal structure when it was annealed at 900 K for long times. At early stage of the annealing, small nuclei form in different places of NP and dissolve for short times. After long times some nuclei form and gather nearby which create the stable clusters in the core of NP. After that the crystal clusters grow in the direction to cover the core and then to spread into the surface of NP. Analyzing the energies of different type atoms, we found that the crystal growth is originated from specific atomic arrangement in the boundary region of crystal clusters.
The study of pressure-induced structural transformations in nanomaterials is both of fundamental and technological importance. Accurate simulations of these transformations are challenging because large length- and time-scales have to be simulated to make contact with experiments whilst retaining the atomic detail for a faithful description. In this thesis, both classical and quantum mechanical techniques are used to model pressure-induced structural transformations in realistic Si, Ge and CdS nanocrystals and comparison made to experiment where possible.
We implement an electronic enthalpy method within the linear-scaling density-functional theory ONETEP code and, after introducing an approach for calibrating the volume definition, investigate the size-dependent pressure-induced amorphisation and polyamorphic transformations in hydrogenated Si and Ge nanocrystals. For the latter, we elucidate the surface-induced amorphisation and the new high-density amorphous metallic Ge phase observed experimentally. We combine this method with the projector-augmented wave and time-dependent density-functional theory methods to study the size and ligand dependence of deformation and optoelectronic properties of CdS nanocrystals with pressure.
We develop a novel classical parametrisation for the simulation of bare and ligated CdS nanocrystals
immersed in a pressure-transmitting medium and investigate their transformation under pressure
using classical molecular dynamics and the metadynamics method for accelerating rare events. The resulting polymorphic transformation and pressure-induced amorphisation are analysed in detail.
Nanoscience is the chemistry subdiscipline to deal with unique and exciting fields, to seek out the new approaches and new applications, to innovatively explore the small world. There is plenty to tackle in the field of nanomaterials and system to effectively solve the anthropogenic problems induced by new technology. This chapter discusses the fundamental properties of nanomaterials and demonstrates their diversified applications, ranging from solar panel used in the space shuttle to atomic reactor (converting CO2 to carbohydrates).
In order to solve such problems as difficult recycling, high cost and hard industrialization realization for the used organic solvent in the preparation process of CdS (CdSe) semiconductor nanoparticles, the CdS and CdSe nanoparticles were prepared by taking the sodium oleate as stabilizer, ethanol as solvent as well as cadmium acetate and thiourea (or NaHSe) as precursors, respectively. The optical properties, crystalline structures, morphologies and dimensions for both CdS and CdSe nanoparticles were characterized with such methods as UV-VIS absorption spectrum, fluorescence spectrum, wide angle x-ray diffraction (WAXD) and transimmision electron spectroscopy (TEM). The results show that with taking the sodium oleate as stabilizer and ethanol as solvent, the CdS and CdSe nanoparticles with homogeneous dimension distribution can be obtained in moderate reaction condition through controlling certain precursor concentration, reaction temperature and reaction time. Therefore, a new way for the synthesis of CdS and CdSe seimconductor nanoparticles in environment-friendly condition can be provided.
Fe nanoparticles have been investigated by means of molecular dynamics simulation. The nucleation and crystal growth is analyzed through the potential energy and number of different types of atoms. The simulation shows that when the amorphous sample is annealed at 900 K, it is crystallized into bcc phase. We found that as the crystal cluster has a size larger than some critical value, the mean potential energy of different types of atoms decreases in following orders: amorphous-atom → surface-crystal atom → crystal-atom. As a result, the crystal cluster is stable and tends to have a nearly spherical shape. Further, it was shown that small nuclei form frequently in the core and rarely in the surface area. After a long annealing time a cluster expands and reaches the critical radius. Then this cluster grows exponentially with times. The fully crystallized sample consists of the core with crystalline structure and surface shell with amorphous porous structure. The Fe nanoparticle has a number of polymorphs which are stable upon annealing at 300 K. We have analyzed the pair radial distribution function (PRDF) for obtained polymorphs. We found that as the fraction of crystal-atoms is less than 0.18, the PRDF is like those of amorphous metal. However, the left sub-peak is higher than right sub-peak when the fraction of crystal-atoms is less than 0.05.
Using molecular dynamics simulation, we have studied the structural evolution of FeB nanoparticle under annealing and the physical properties of its polymorphs such as crystalline, amorphous and mixed samples. The main focus of present work is the crystallization mechanism and the local structure of polymorphs of FeB nanoparticle. The simulation result shows that the amorphous sample undergoes the crystallization via the nucleation mechanism. During the crystallization, B atoms move out the places where the Fe crystal locates, and diffuse to the boundary region of Fe crystal. The crystal growth proceeds when this boundary region attains specific properties which are defined by the fraction of B atoms and the energies of AB-atoms and CB-atoms. Further our study indicates that unlike amorphous sample, the crystalline and mixed samples consist of three distinct parts including Fe crystalline and two FeB amorphous parts (B-poor and B-rich amorphous part). The different polymorphs of FeB nanoparticle differ in the local structure, size of Fe crystal and energies of different type atoms.
Ultrafast exciton dynamics of aligned polycrystalline nanorod arrays composed of CdSe or CdSe/TiO2 grown on conductive glass substrates using oblique angle deposition/codeposition have been studied using femtosecond transient absorption (TA) spectroscopy. Scanning electron microscopy images show that the morphology of the two samples are comparable in height, width, and tilt angle. X-ray diffraction and Raman spectroscopy indicate that the as-deposited CdSe nanorod arrays are in the hexagonal phase, while the TiO2 is amorphous. In the TA studies, a pump wavelength of 580 nm was used to determine the exciton lifetimes of CdSe in the two samples. Transient bleach dynamics probed at 695 nm can be fit with triple exponential functions with lifetimes of 7 ps, 84 ps, and 1.0 ns for CdSe nanorods versus 0.5 ps, 3 ps, and 24 ps for the CdSe/TiO2 composite-nanorods. These lifetimes are independent of the pump power, indicating that nonlinear processes are not involved. For CdSe nanorods, the two fast decays are mainly due to nonradiative electron–hole recombination or exciton relaxation mediated by trap states. The overall much faster decay in CdSe/TiO2 nanorods is due to electron transfer from the conduction band of CdSe to the conduction band of TiO2. The electron injection rate from CdSe into TiO2 was calculated to be 1.7 × 1011 s–1 based on the average lifetime measured for CdSe with and without TiO2. This very high rate of electron injection is attributed to the large interfacial area and strong coupling between the two materials in CdSe/TiO2 composite-nanorods. Such strongly coupled semiconductor–metal oxide heterostructures are desired for applications in solar energy conversion.
We present a joint experimental and theoretical study of the early stage dynamics of photoexcited charges in a prototypical organic/inorganic interface. By using femtosecond pump–probe experiments we compared the photophysic of a layer-by-layer hybrid structure obtained by alternating CdSe nanocrystals and poly(p-styrenesulphonic acid) and the same CdSe nanocrystals capped with hexadecylamine and stearic acid diluted in solutions. While in the LBL structure it is clear the appearance of a long-lived charged state, no evidence of this is instead found in the diluted solutions. Density functional calculations indicate that these states are localized close to the nanoparticle surface, and that electrons and holes are separated across the hybrid interface, pointing out the effects of surfactant capping molecules on the optoelectronic properties of the interface. Our combined approach, allowing for unique access to the photoexcited electronic structure, opens the possibility to the fine tailoring of hybrid organic/semiconducting layers for photovoltaic applications.
The data obtained by both experiments and computer simulations concerning the amorphous nanoparticles for decades including methods of synthesis, characterization, structural properties, atomic mechanism of a glass formation in nanoparticles, crystallization of the amorphous nanoparticles, physico-chemical properties (i.e. catalytic, optical, thermodynamic, magnetic, bioactivity and other properties) and various applications in science and technology have been reviewed. Amorphous nanoparticles coated with different surfactants are also reviewed as an extension in this direction. Much attention is paid to the pressure-induced polyamorphism of the amorphous nanoparticles or amorphization of the nanocrystalline counterparts. We also introduce here nanocomposites and nanofluids containing amorphous nanoparticles. Overall, amorphous nanoparticles exhibit a disordered structure different from that of corresponding bulks or from that of the nanocrystalline counterparts. Therefore, amorphous nanoparticles can have unique physico-chemical properties differed from those of the crystalline counterparts leading to their potential applications in science and technology.
We introduce a novel method to improve the device performance of P3HT:CdSe hybrid solar cells by using selenourea (SeU) for ligand exchange. SeU induces interconnection of CdSe nanorods in the nanoscale range without severe aggregation. The power conversion efficiency of the devices with SeU is improved from 1.71% to 2.63% due to efficient charge transport through interconnected CdSe nanorods.
Compared to information on nanocrystals, that on amorphous nanosolids is on the whole much less organized. On the other hand, growth of structural data in recent years on the latter, that deal with the range of atomic order (short range order and beyond), coordinations of core and surface atoms and similar aspects in amorphous nanoparticles through computer simulation and other techniques, has been very impressive. Similar generation of information is also true for physical phenomena like crystallization and melting. Finally, interesting properties revealed through experimentations point toward important applications. The present article makes a brief survey of these areas and attempts at reaching certain conclusions mostly specific for amorphous nanostructures with respect to the crystalline counterparts. The article analyzes the structural data to try and explain different properties of amorphous nanosolids and also their position in the applications scenario.
Worm-like palladium nanoparticles are formed in the reverse micellar organic phase by means of chemical reduction in the water pools of reverse micelles. Water-soluble, spherical Pd nanocrystals are extracted from the worm-like Pd nanoparticles through the digestive ripening behavior of mercaptocarboxylic acids as stabilizing agents. It was found that during the reaction the production of spheres occurs in parallel with the formation of worm-like structure by association of the spheres on increasing the reaction time and the crystallization process occurs even during the extraction.
A titanium chloromethoxide solution was prepared by reacting TiCl4 with methanol, followed by water addition. The starting solutions were characterized by Fourier Transform Infrared (FTIR)
spectroscopy, evidencing that the in situ generated water results in early hydrolysis of the chloroalkoxide. The solution
was reacted with molten dodecylamine at room temperature, obtaining a white slurry of amorphous titania nanoparticles. Stable,
redispersible TiO2 nanocrystals could be prepared by subsequent solvothermal treatment in oleic acid at 250 °C. The use of oleic acid was essential
for obtaining crystalline structures, while other surfactants prevented crystallization. The nanocrystals were characterized
by X-ray Diffraction and Transmission Electron Microscopy, confirming the formation of anatase TiO2 nanocrystals with a mean size of 3.3 nm. The TiO2 nanocrystals were used for fabricating gas-sensing devices, which were tested towards ethanol vapors. The initial small size
of the nanocrystals, and the limited size growth during the high-temperature sensor operation, result in remarkable sensing
performances if compared with bulk titania sensors.
We report on the synthesis and optical and structural characterization of ultrasmall (<2 nm) CdS nanoparticles with a narrow size distribution of 10% prepared in aqueous and alcohol solutions of polyethyleneimine (PEI). The PEI-stabilized CdS nanoparticles reveal a structured absorption band with the first excitonic maximum at 3.5 eV and broad-band photoluminescence with quantum yields of 12−14% in water, 18−20% in ethanol (80 vol %), and up to 60−70% in solid PEI films at room temperature. The nature of the photoluminescence was studied by using the time- and wavelength-dependent emission measurements. The role of precursor cadmium(II)−PEI complex in the formation of uniform and ultrasmall luminescent CdS nanoparticles, as well as the dynamic emission quenching by water, are discussed. A study of the photochemical properties of PEI-stabilized CdS nanoparticles both under continuous and nanosecond pulse illumination showed excellent stability of solid PEI films incorporating CdS nanoparticles toward UV illumination and reductive character of the CdS nanoparticle photocorrosion in aqueous solutions.
The decomposition of metal (Ti, Zr, Sn) alkoxides at 250 degrees C in a solution of tetradecene and dodecylamine resulted in the formation of ultra-small (1-2 nm) oxide nanoparticles. The nanoparticles showed unusual structural properties. The high-pressure orthorhombic phase was found for SnO(2). The as-synthesized ZrO(2) nanoparticles were only partially crystallized. It was possible to observe their in situ crystallization under the TEM beam. The TiO(2) nanoparticles appeared amorphous, with only a few nanocrystals dispersed in the sample. The temperature evolution of the samples was investigated in situ by Raman spectroscopy. All SnO(2) was converted to stable cassiterite at 350 degrees C. TiO(2) and ZrO(2) samples displayed phase stability reversal over a broad range of temperatures. (C) 2010 Elsevier B.V. All rights reserved.
Metal ferrite (MFe(2)O(4), with M = Fe, Mn, Co Ni Cu, Zn) nanoparticles were synthesized by processing metal oxide sols in a coordinating environment The sols were prepared by forced hydrolysis of the starting metal nitrates, in the presence of acetylacetone for avoiding precipitation Two different processing routes were investigated In the first, the sol was injected into a hot (160 degrees C) solution of dodecylamine in tetradecene In the second route the Injection environment was constituted by pure dodec ylamine heated at the same temperature The precipitate from the first route was heat-treated in air at various temperatures, from 200 to 500 degrees C The redispersible nanoparticles from the second route were annealed in oleylamine at temperatures up to 220 degrees C In the first case, crystallization was obtained only after heat-treatment at 500 degrees C, while 220 degrees C was sufficient for crystallizing 1 he nanoparticles dispersed in oleylamine The samples from the two routes were investigated by X-ray diffraction and transmission electron microscopy/electron energy loss spectroscopy in the case system of NiFe(2)O(4) The product from the first route, after heating at 200 degrees C, was a disordered material, with a broad size distribution of aggregates and Ni depletion regions The product from the second route was constituted by discrete nanoparticles with the correct cation stoichiometry The interpretation of the results allowed concluding that obtaining simple structural reorganization in nanosized volumes is a key factor for crystallization under mild conditions
Modulation of Pd nanoparticle (NP) crystallinity is achieved by switching the surfactants of different binding strengths. Pd NPs synthesized in the presence of weak binding surfactants such as oleylamine possess polyhedral shapes and a polycrystalline nature. When oleylamine is substituted by trioctylphosphine, a much stronger binding surfactant, the particles become spherical and their crystallinity decreases significantly. Moreover, the Pd NPs reconvert their polycrystalline structure when the surfactant is switched back to oleylamine. Through control experiments and molecular dynamics simulation, we propose that this unusual nanocrystallinity transition induced by surfactant exchange was resulted from a counterbalance between the surfactant binding energy and the nanocrystal adhesive energy. The findings represent a novel postsynthetic approach to tailoring the structure and corresponding functional performance of nanomaterials.
The design and performance of a compact fluorescense XAFS apparatus equipped with a microfluidic cell for in situ studies of nanoparticles are described. CdSe nanoparticles were prepared by solution reaction starting from trioctylphosphine-Se. Time-resolved experiments were performed by precisely controlling the reactor coordinates (x,y), allowing the synchrotron X-ray beam to travel along a reactor channel, covering nucleation and initial growth of nanoparticles. Detailed analysis of EXAFS data combined with UV-vis spectra allow reliable estimation of particle size and density in the initial growth that cannot be accessible by conventional optical techniques based on a long-range order. The Se K-XANES spectra are interpreted by multi-scattering calculations providing bond formation kinetics consistent with the EXAFS data.
The pressure-induced structural transformation in cadmium selenide is studied with the isothermal-isobaric molecular-dynamics method and electronic-structure calculations based on the density-functional theory. The reversible transformation between the fourfold-coordinated wurtzite structure and the sixfold-coordinated rocksalt structure is successfully reproduced in the molecular-dynamics simulations, in which atomistic transition mechanisms including the existence of a metastable state as well as barrier states along the transition paths are observed. Accurate density-functional calculations confirm these transition paths. It is shown that there are at least three transition paths, which are characterized by atomic shifts in the (0001) plane of the wurtzite structure. The energy barrier for the transformation is found to be about 0.13 eV∕pair and is almost independent of the paths.
We describe a high-pressure x-ray diffraction (XRD) study of the compressibility of several samples of ZnS nanoparticles. The nanoparticles were synthesized with a range of sizes and surface chemical treatments in order to identify the factors that determine nanoparticle compressibility. Refinement of the XRD data revealed that all ZnS nanoparticles in the nominally cubic (sphalerite) phase exhibited a previously unobserved structural distortion under ambient conditions that exhibited, in addition, a dependence on pressure. Our results show that the compressibility of ZnS nanoparticles increases substantially as the particle size decreases, and we propose an interpretation based upon the available mechanisms of structural compliance in nanoscale vs bulk materials.
Dense powder of nanocrystalline ZnO has been recovered at ambient conditions in the metastable cubic structure after a heat treatment at high pressure (15 GPa and 550 K). Combined x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) experiments have been performed to probe both long-range order and local crystallographic structure of the recovered sample. Within uncertainty of these techniques (about 5%), all the crystallites are found to adopt the NaCl structure. From the analysis of XRD and XAS spectra, the cell volume per chemical formula unit is found to be 19.57(1) and 19.60(3) Å3, respectively, in very good agreement with the zero-pressure extrapolation of previously published high-pressure data. © 2002 American Institute of Physics.
A model for size-dependent surface energy of nanocrystals, free of any adjustable parameter, has been established based on a previous model for the size-dependent cohesive energy. The surface energy falls as the size of crystals decreases to several nanometers while the surface energy ratio between different facets is size-independent and is equal to the corresponding bulk ratio. The predictions agree with experimental or theoretical results for beryllium, magnesium, sodium, aluminum, and gold.
The thermodynamic behaviour of small particles differs from that of the bulk material by the free energy term gammaA--the product of the surface (or interfacial) free energy and the surface (or interfacial) area. When the surfaces of polymorphs of the same material possess different interfacial free energies, a change in phase stability can occur with decreasing particle size. Here we describe a nanoparticle system that undergoes structural changes in response to changes in the surface environment rather than particle size. ZnS nanoparticles (average diameter 3 nm) were synthesized in methanol and found to exhibit a reversible structural transformation accompanying methanol desorption, indicating that the particles readily adopt minimum energy structural configurations. The binding of water to the as-formed particles at room temperature leads to a dramatic structural modification, significantly reducing distortions of the surface and interior to generate a structure close to that of sphalerite (tetrahedrally coordinated cubic ZnS). These findings suggest a route for post-synthesis control of nanoparticle structure and the potential use of the nanoparticle structural state as an environmental sensor. Furthermore, the results imply that the structure and reactivity of nanoparticles at planetary surfaces, in interplanetary dust and in the biosphere, will depend on both particle size and the nature of the surrounding molecules.
Nanoparticles may contain unusual forms of structural disorder that can substantially modify materials properties and thus
cannot solely be considered as small pieces of bulk material. We have developed a method to quantify intermediate-range order
in 3.4-nanometer-diameter zinc sulfide nanoparticles and show that structural coherence is lost over distances beyond 2nanometers.
The zinc-sulfur Einstein vibration frequency in the nanoparticles is substantially higher than that in the bulk zinc sulfide,
implying structural stiffening. This cannot be explained by the observed 1% radial compression and must be primarily due to
inhomogeneous internal strain caused by competing relaxations from an irregular surface. The methods developed here are generally
applicable to the characterization of nanoscale solids, many of which may exhibit complex disorder and strain.
Experiments have shown that thermal stability of nanoscale titania is significantly enhanced by the presence of low concentration yttrium dopants, but the mechanism of this effect is unclear. We present extended x-ray fine structure and wide-angle x-ray scattering measurements showing that yttrium is not incorporated in the nanoparticle interior but forms yttrium-oxygen clusters at the nanoparticle surfaces. The surface clusters modify the interfacial free energy, affecting the unit cell parameters, strongly inhibiting nanoparticle growth, and stabilizing the anatase phase up to 700 degrees C. Molecular dynamics calculations reproduced the experimentally observed Y-O bond lengths in surface yttrium clusters and predict a substantial lowering in anatase surface energy, in agreement with the inferred mechanism for suppression of growth and phase transformation.
We report the observation of size dependent structural disorder by x-ray absorption near-edge spectroscopy (XANES) in InAs and CdSe nanocrystals 17{endash}80thinspthinsp{Angstrom} in diameter. XANES of the In and Cd M{sub 4,5} edges yields features that are sharp for the bulk solid but broaden considerably as the size of the particle decreases. FEFF7 multiple-scattering simulations reproduce the size dependent broadening of the spectra if a bulklike surface reconstruction of a spherical nanocrystal model is included. This illustrates that XANES is sensitive to the structure of the entire nanocrystal including the surface. {copyright} {ital 1999} {ital The American Physical Society }
CdSe thin films deposited chemically on glass substrates for 4, 8, and 16 h, and subsequently annealed at 400 °C for 1 h, have been studied by a combination of spectroscopic (photoluminescence and Raman scattering) and structure-determining (x-ray diffraction and atomic force microscopy) techniques. Due to a size distribution of constituent grains, photoluminescence spectra of the as-deposited films show weak but broad bands at ∼ 2.2 eV (strongly confined band) and ∼ 1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at ∼ 1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at ∼ 250 cm−1 in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show a diffraction peak at 2θ = 13° from the (001) plane. The improvement in crystallinity of the films as observed by atomic force microscopy and photoluminescence techniques, the appearance of (001) reflection in the x-ray diffraction pattern, the disappearance of surface-optic Raman mode, and the enhancement of weakly confined band—all as a consequence of annealing—have been discussed and correlated with each other. A film deposition mechanism has been described, which explains the origin of the simultaneous existence of strong and weak quantum confinement effects; the significance this observation in the development of high efficiency photovoltaic solar cells has been emphasized.
Nanocrystallites of tungsten oxide samples of 2, 4, 16, 35 and 60 nm of diameter were prepared by cryosol and pyrosol techniques. The pressure- and temperature-induced phase transitions of these samples were monitored by Raman spectrometry from 0.1 MPa to 34 GPa and from 77 to 1200 K. The tetragonal (α)–orthorhombic (β)–monoclinic (γ) transitions in these nanometric samples are strongly downshifted in temperature by comparison with the bulk WO3. For instance, the tetragonal phase which exists above 1171 K for the bulk tungsten oxide can be stabilized at 700 K for the 35 nm sample. In the same way, the monoclinic P21/n-monoclinic P21/c high-pressure-induced transition is slightly shifted from 0.1 GPa to a higher pressure (1.5 GPa). The discussion of these transition-line shifts is based on thermodynamic considerations in which the surface energy of crystallites plays an important role.
Resonant Raman scattering has been studied in trigonal Se at low temperatures in the region of its indirect and direct excitonic absorption edges. Whereas no enhancement in Raman cross sections was observed at the indirect absorption edge, strong dispersion was found in the region of the direct excitons. In the vicinity of the direct absorption edge Raman cross sections of one-phonon modes decreased monotonically with increase in photon energy in all three scattering configurations studied. This monotonic decrease in Raman cross sections was explained qualitatively by resonant cancellation between a constant background and a dispersive term due to the 2.20-eV peak in the reflectivity spectrum of Se. Resonant enhancements were observed at the direct excitons only in the scattering configuration where the incident and scattered photons are both polarized perpendicular to the c axis. The enhancements are explained quantitatively by a simple model involving two direct excitons of Se as resonant intermediate states. We have also determined directly the longitudinal and transverse mode splitting of the low-energy E mode in trigonal Se as 7 ± 2 cm-1.
High-pressure behaviors of wurtzite-type hexagonal CoO nanocrystals were investigated by in situ high-pressure synchrotron radiation X-ray diffraction measurements up to 57.4 GPa at ambient temperature. It is found that bulk modulus of the hexagonal CoO phase is about 115 GPa at zero pressure. During compression, the hexagonal CoO phase transfers into rocksalt-type cubic phase in the pressure range of 0.8−6.9 GPa. The volume collapse accompanied by the transition was estimated to be about 20%. This is irreversibly phase transformation; that is, the cubic CoO phase remains after pressure release. Based on the data of peak width vs pressure, a cubic-to-rhombohedral phase transition was detected for the nanocrystalline cubic CoO phase with the transition pressure of about 36 GPa, lower than 43 GPa for bulk cubic CoO phase. The bulk modulus of the nanocrystalline cubic CoO phase of about 258 GPa is larger than 180 GPa for the corresponding bulk cubic CoO phase.
The surface free energy of small particles in an aqueous solution consists of the electrostatic energy of charged surfaces and the interfacial energy. For nanoparticles in an aqueous solution, the two terms can be modified by solution chemistry and be manipulated to control phase stability and transformation kinetics. Here we show that the phase stability of titania (TiO2) nanoparticles strongly depends on the solution pH. At small sizes, rutile is stabilized relative to anatase in very acidic solutions, whereas in very basic solutions anatase is stabilized relative to rutile and brookite. Rutile is the stable phase at large particle sizes regardless of pH. These results indicate that the activity of potential determining ions (protons or hydroxyl groups) is a factor that can determine the phase stability of nanoparticulate titania in aqueous solutions at pH values far from the point of zero charge of titania. The phase transformation proceeds via a dissolution−precipitation mechanism under hydrothermal conditions.
The thermodynamic amount of the pressure-induced solid transition between the wurtzite phase and the rock-salt phase and the melting thermodynamic amounts of the metastable rock-salt phase for CdSe nanocrystals are considered based on known thermodynamic data of the wurtzite phase. Moreover, the static hysteresis width of the above solid transition is also determined. The predictions are found to be reasonable and are in agreement with the present theoretical and experimental results.
High-quality colloidal mercury sulfide quantum dots (QDs) are synthesized at room temperature using a strategy combining the effects of strongly binding Hg(II) ligands and metal/chalcogen precursor phase separation. This combination prevents both the rapid precipitation of bulk HgS in preparations involving only weak Hg(II) ligands and the reduction of mercury that takes place when only strongly binding ligands are used to slow the growth kinetics. Both the linear absorption and complementary band edge emission of the synthesized HgS QDs exhibit narrow, size-dependent transitions between 500 and 800 nm for sizes ranging from 1 to 5 nm in diameter. The metastable zinc blende phase of HgS is verified by wide-angle X-ray diffraction experiments and suggests potentially large tunable band edges if larger HgS nanocrystals that approach the bulk (zero energy) gap can be made. Growth of HgS QDs can be arrested by subsequent addition of Cd or Zn to the surface, after which the QDs can be stabilized with long-chain thiols or amines.
We show that metastable rocksalt CdSe nanocrystals can persist at ambient pressure depending on the physical size of the particle. The size-dependence of the hysteresis loop was measured for the solid−solid transition in CdSe nanocrystals, between four- and six-coordinate structures. A systematic shift of the entire hysteresis loop to lower pressure results in a threshold size of 11 nm for ambient metastability of the six-coordinate rocksalt structure. Smaller nanocrystals transform back to the four-coordinate structure as occurs in the CdSe bulk solid. Surface energy contributions are used to explain the shift. The results have important implications for the optimum synthesis of metastable nanocrystal solids under ambient conditions.
Surface Raman scattering is used to study self-assembled monolayers formed from a series of 1-alkanethiols (1-butanethiol, 1-dodecanethiol, 1-octadecanethiol) at both electrochemically roughened and mechanically polished polycrystalline Ag electrodes. The spectra obtained at both surfaces are similar in all spectral regions. Defect structure in these films is investigated using the relative amounts of trans and gauche conformers in the nu(C-S) and nu(C-C) frequency regions. These monolayer films are most ordered in the cases of 1-butanethiol and 1-octadecanethiol and least ordered in the case of 1-dodecanethiol. This behavior correlates with the ordering observed in the bulk 1-alkanethiols. Surface selection rules are used to determine molecular orientation at Ag.
CdS crystallites of 4-nm diameter have been studied under high pressure up to 10 GPa by optical absorption and Raman scattering. The solid-solid phase transition from the zinc blende to the rock salt phase is observed at pressures far in excess of the bulk phase transition pressure of 3 GPa. The pressure of the phase transition depends on the nature of the moiety used to derivatize the surface. In addition, because the compression of the lattice with pressure is the same as in the bulk crystal, it is possible to observe the dependence of the zinc blende crystallite properties up to pressures far higher than in the bulk. The elevated phase transition pressure and its dependence on the surface stabilizer can be explained by a higher value of the surface tension for the rock salt phase nanocrystals compared to the zinc blende.
The temperature dependence of a kinetic rate constant for a phase transformation is usually described by the Arrhenius equation, which comprises a preexponential factor multiplied by an exponential term involving the activation energy and temperature. In this work we show that particle size is another factor that is needed in the description of kinetics of phase transformations in nanoparticles. For the phase transformation in nanocrystalline titania (TiO2) proceeding via nucleation at particle−particle contacts, the activation energy varies slightly with particle size but the preexponential factor is inversely proportional to the approximately fourth power of the particle size. We attribute the large preexponential factor primarily to the high concentration of nucleation sites at particle−particle interfaces in nanomaterials compared to bulk materials. We proposed a kinetic equation that incorporates the dependence of the rate constant on the particle size for phase transformation via interface nucleation in nanoparticles.
X-ray diffraction and Raman data on the pressure induced phase transitions of a nanometric zirconia, ZrO2, are analyzed via a classical phenomenological Landau approach of the bulk. It is concluded that the initial tetragonal structure (D
4h
15), which is a metastable bulk state of zirconia at ambient conditions, evolves continuously towards the ideal cubic fluorite structure (O
h
5) via an intermediate tetragonal form (D
4h
14). The proposed phenomenological model describes consistently all experimental peculiarities, including the hybridization and softening of the low-frequency Raman active modes along with lattice-parameter anomalies.
The pressure–temperature phase diagrams of different zirconia samples prepared by oxidation of Zircaloy-4 and Zr–1%Nb–0.12O alloys were monitored by Raman spectrometry from 0.1 MPa to 12 GPa and from 300 to 640 K. These new diagrams show that the monoclinic–tetragonal equilibrium line is strongly downshifted in temperature compared to literature measurements performed on usual polycrystalline zirconia. In addition, the monoclinic–orthorhombic equilibrium line is slightly shifted to higher pressure (i.e. 6 GPa). The crystallite sizes smaller than 30 nm, are thought to be responsible for these equilibrium line displacements. The tetragonal phase obtained in temperature under high pressure can be quenched at room temperature, if the pressure is maintained, and it is destabilised and transforms completely into monoclinic phase if the pressure is released. These results confirm that coupled effects of stress, temperature and nanosized grain are responsible for the formation of the tetragonal phase near the metal/oxide interface during the oxidation of zirconium-based alloys.
Cubic CdSe thin films of approximately 2000 Å thickness have been prepared onto glass substrates by chemical bath deposition. The samples were annealed in an Ar+Se2 atmosphere at normal pressure for 30 h at different temperatures in the range of 50–500°C, in order to perform the crystalline phase transformation from cubic zincblende (ZB) to the hexagonal wurtzite (W) structure. The characterization of samples included both optical absorption and X-ray diffraction analyses. The optical absorption spectra allowed to calculate the energy band-gap (Eg) values and, hence, the evolution of Eg in the thermally treated samples through the transformation from the cubic crystalline phase to the hexagonal phase. The X-ray diffraction spectra also showed the complete microstructural transformation from as-grown cubic samples up to the entire hexagonal lattice for samples with higher annealing. The critical point of the ZB→W transformation is proposed to occur at 355±25°C.
Amorphous CdSe nanoparticles were prepared by a room-temperature reaction between selenourea and cadmium nitrate and subsequently crystallized by heating at 145 degrees C in pyridine. Prolonged heating at this temperature did not result in nanocrystal growth, while the addition, before the heating step, of very small amounts of SeO2 dissolved in tetrahydrofuran triggered the dissolution of the small initial nanocrystals and the formation of larger particles. The process is controlled by the relative concentrations of SeO2 and of the capping agent (decanethiol) used in the synthesis of the starting particles. The occurring of an Ostwald ripening mechanism during the treatment with SeO2 was demonstrated by the evolution of the optical absorption spectra, where the sharp excitonic peaks of the initial small nanocrystals progressively disappeared giving rise to broad peaks at longer wavelengths. The results are interpreted as a fast etching of the nanocrystal surfaces by the formation of Se redox couples, enhancing the dissolution rate of the small nanocrystals and providing new monomers for the growth of larger, crystalline particles. The whole process is interpreted as an example of Ostwald ripening triggered by a moving solubility perturbation.
Extremely small 1.4-nm size mercaptoethanol-stabilized ZnS clusters have been synthesized with narrow size distribution. The structure of these clusters was studied by wide-angle X-ray scattering. The scattering curves were compared with the calculated scattered intensity of a variety of model clusters (ZnS)N and different defect types via Debye functions. In the as-received state the pattern is best described by a fragment of the zinc blende lattice, with N ≈ 30, and a defective stacking of three to four (111) planes. A large improvement of the simulation is gained by introducing liquidlike disorder to the model structure. This raises the unanswered question of a 'real' liquid state of these small clusters at room temperature. The cluster matrix is thermally stable to 583 K. Above this temperature the primary cluster coalesce to form larger particles. Annealed at 1013 K the particles grow to >4.0 nm with a highly defective zinc blende structure.
The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases.
The kinetics of solid-solid phase transitions are explored using pressure-induced structural transformations in Si nanocrystals. In agreement with the predictions of homogeneous deformation theories, large elevations in phase transition pressure are observed in nanocrystals as compared to bulk Si, and high pressure x-ray diffraction peak widths indicate an overall change in nanocrystal shape upon transformation. In addition, unlike the BC8 phase recovered in bulk Si, amorphous Si nanoclusters are obtained upon release of pressure, providing an example of kinetic size control over solid phases. {copyright} {ital 1996 The American Physical Society.}
X-ray diffraction was used to monitor the structure of 45 A diameter CdSe nanocrystals as they transformed repeatedly between fourfold and sixfold coordinated crystal structures. Simulations of the diffraction patterns reveal that a shape change occurs as the crystals transform. They also show that stacking faults are generated in the transition from the high- to the low-pressure phase. The shape change and stacking fault generation place significant constraints on the possible microscopic mechanism of the phase transition.
The transition between four- and six-coordinate structures in CdSe nanocrystals displays simple transition kinetics as compared with the extended solid, and we determined activation volumes from the pressure dependence of the relaxation times. Our measurements indicate that the transformation takes place by a nucleation mechanism and place strong constraints on the type of microscopic motions that lead to the transformation. The type of analysis presented here is difficult for extended solids, which transform by complicated kinetics and involve ill-defined domain volumes. Solids patterned on the nanoscale may prove to be powerful models for the general study of structural transitions in small systems, as well as in extended solids.
The structure of 3 nm ZnS nanoparticles differs from that of bulk ZnS and is shown to vary with the particle aggregation state. Dispersed or weakly aggregated nanoparticles in suspension have a more distorted internal structure than strongly aggregated nanoparticles. Reversible switching between distorted and crystalline structures can be induced by changing the aggregation state via slow drying and ultrasonic agitation. The transformation was analyzed using pair distribution function data from wide angle x-ray diffraction and the aggregation state monitored via small angle x-ray scattering. Molecular modeling provides insight into particle-particle interactions that induce the structural changes. The reversible nature also implies a low activation energy of nanoparticle transformation and indicates that distorted nanoparticles are not trapped in a metastable state.
In this work, we examine the phase stability of both uncoated and alumina-coated zirconia nanoparticles using in-situ X-ray diffraction. By tracking structural changes in these particles, we seek to understand how changing interfacial bonding affects the kinetics of amorphous zirconia crystallization and the kinetics of grain growth in both initially amorphous and initially crystalline zirconia nanocrystals. Activation energies associated with crystallization are calculated using nonisothermal kinetic methods. The crystallization of the uncoated amorphous zirconia colloids has an activation energy of 117 +/- 13 kJ/mol, while that for the alumina-coated amorphous colloids is 185 +/- 28 kJ/mol. This increase in activation energy is attributed to inhibition of atomic rearrangement imparted by the alumina coating. The kinetics of grain growth are also studied with nonisothermal kinetic methods. The alumina coating again dramatically affects the activation energies. For colloids that were coated with alumina when they were in an amorphous structure, the coating imparts a 5x increase in the activation energy for grain growth (33 +/- 8 versus 150 +/- 30 kJ/mol). This increase shows that the alumina coating inhibits zirconia cores from coarsening. When the colloids are synthesized in the tetragonal phase and then coated with alumina, the effect of surface coating on coarsening kinetics is even more dramatic. In this case, a 10x increase in activation energies, from 28 +/- 3 kJ/mol for the uncoated particles to 300 +/- 25 kJ/mol for the alumina-coated crystallites, is found. The results show that one can alter phase stability in colloidal systems by using surface coatings and interfacial energy to dramatically change the kinetic barriers to structural rearrangement.
Pressure-induced structural transformations in spherical and faceted gallium arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular-dynamics approach. The results show that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size, and all nanocrystals undergo nonuniform deformation during the transformation. Spherical nanocrystals above a critical diameter >/=44 A transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. The structure of transformed nanocrystals shows that domain orientation and strain relative to the initial zinc blende lattice are not equivalent. These observations may have implications in interpreting the experimental x-ray line shapes from transformed nanocrystals.
Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 degrees C at low pressure, sphalerite transforms to wurtzite at 225 degrees C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite-wurtzite interface reaches approximately 22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite-wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.
Investigation of the growth of CdSe nanocrystals ( approximately 160 atoms) to the uniquely stable size of 2 nm allows the monitoring of the crystallization process in semiconductor quantum dots. By using a combination of optical techniques, high-resolution transmission electron microscopy (HRTEM), and powder X-ray diffractometry (XRD), new phenomena were explored during the CdSe nanocrystal growth process, which involved significant morphological reconstruction and crystallization of the initially formed amorphous nanoparticles. During the crystallization, the absorption onset of the CdSe quantum dots blue shifted toward higher energies at 3 eV (414 nm), while the photoluminescence red shifted to lower energies. Furthermore, an apparent increasing Stokes shift was observed during the formation of small CdSe nanoparticles. On the other hand, the photoluminescence excitation spectra showed constant features over the reaction time. Additionally, results from HRTEM and XRD studies show that the CdSe nanoparticles were amorphous at early reaction stages and became better crystallized after longer reaction times, while the particle size remained the same during the crystallization process. These observations demonstrate the important role of the surface on the optical properties of small CdSe quantum dots and facilitated the spectroscopic monitoring of the crystallization process in quantum dots.
High purity, spherical anatase nanocrystals were prepared by a modified sol-gel method. Mixing of anhydrous TiCl(4) with ethanol at about 0 degrees C yielded a yellowish sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 degrees C for 3 days. This synthesis route eliminates the presence of fine seeds of the nanoscale brookite phase that frequently occurs in low-temperature formation reactions and also significantly retards the phase transformation to rutile at high temperatures. Heating the as-is 7.7 nm anatase for 2 h at temperatures up to 600 degrees C leads to an increase in grain size of the anatase nanoparticles to 32 nm. By varying the calcination time from 2 to 48 h at 300 degrees C, the particle size could be controlled between 12 and 15.3 nm. The grain growth kinetics of anatase nanoparticles was found to follow the equation, D(2) - D(0)(2) = k(0)t(m)e((-)(E)(a)/(RT)) with a time exponent m = 0.286(+/-9) and an activation energy of E(a) = 32 +/- 2 kJ x mol(-)(1). Thermogravimetric analysis in combination with infrared and X-ray photoemission spectroscopies has shown the anatase nanocrystals at different sizes to be composed of an interior anatase lattice with surfaces that are hydrogen-bonded to a wide set of energetically nonequivalent groups. With a decrease in particle size, the anatase lattice volume contracts, while the surface hydration increases. The removal of the surface hydration layers causes coarsening of the nanoparticles.
Combined small-angle and high energy wide-angle X-ray scattering measurements of nanoparticle size and structure permit interior strain and disorder to be observed directly in the real-space pair distribution function (PDF). PDF analysis showed that samples of ZnS nanoparticles with similar mean diameters (3.2-3.6 nm) but synthesized and treated with different low-temperature procedures possess a dramatic range of interior disorder. We used Fourier transform infrared spectroscopy to detect the surface species and the nature of surface chemical interactions. Our results suggest that there is a direct correlation between the strength of surface-ligand interactions and interior crystallinity.
Hexagonal CoO nanocrystals are coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation and stability property. Structural stability and phase transformation of the hexagonal CoO phase have been investigated by X-ray powder diffraction with Rietveld refinement, transmission electron microscopy, X-ray absorption fine structure, and differential scanning calorimeter. It is found that the hexagonal CoO phase is a metastable phase, which increases its grain size from 50 to 250 nm for refluxing times from 1 to 6 h at 200 degrees C. After 12 h, cubic-structured CoO grains with an average grain size of 20 nm are observed, which spread around big hexagonal CoO grains. After about 24 h, only the cubic CoO phase with an average grain size of 25 nm is detected. The onset temperature of hexagonal-to-cubic phase transformation in CoO is estimated to be 378 degrees C by DSC, using a heating rate of 20 deg/min. The results obtained indicate that the hexagonal-to-cubic phase transformation in nanocrystalline CoO is by nucleation and growth mechanism, starting from the surface to the center of the hexagonal grains.
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