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Coherent electron–nuclear coupling in oligothiophene molecular wires
Abstract
In molecular electronics individual molecules serve as electronic devices. In these systems, electron–vibron (e–ν) coupling can be expected to lead to new physical phenomena and potential device functions1, 2, 3. In previous studies of molecular wires, the e–ν coupling occurred as a result of the well-known Franck–Condon principle, for which the Born–Oppenheimer approximation holds. This means that after a vibronic excitation, the electrons and the vibrations evolve independently from each other. Here we show that this simple picture changes markedly when two electronic levels in a molecule are coupled by a molecular vibration4, 5. In molecular wires we observe a non-Born–Oppenheimer regime, for which a coherent coupling of electronic and nuclear motion emerges6. This phenomenon should occur in all systems with strong electron–vibration coupling and an electronic level spacing of the order of vibrational energies. The coherent coupling of electronic and nuclear motion could be used to implement mechanical control of electron transport in molecular electronics
... Ultrathin layers of conventional insulating materials, for example, alkali-halides [137][138][139], metal-oxides [22,76,140,141], metal-nitrides [142,143], passivated semiconductors [144,145] have been used to study the properties of single atoms, single molecules, and various aspects of molecular electronics in general. Usage of twodimensional films of noble gases [146,147], molecules [148][149][150][151][152] also serve similar purpose of decoupling molecules from the metal surface. ...
... magnetic moment [81], fluorescence [22]). In addition, it helps more precise tunneling spectroscopy experiments as the increasing electron residence time leads to sharper molecular resonance peaks [151,153,154], and the observation of vibronics [147,153] in the STM/STS measurements. In a seminal experiment, Repp et al. [137] demonstrated that few layers of NaCl is sufficient to electronically decouple pentacene molecules from the underlying Cu(111) surface. ...
... The decoupling by hBN results in sufficiently narrow widths of the molecular resonances which allows resolving vibronic features in the spectra. The vibronic peaks are the inelastic features resulting from elastic tunneling through one of the molecular resonances while simultaneously exciting a molecular vibration [147,153]. This is different from inelastic electron tunneling spectroscopy which correspond to inelastic scattering in non-resonant tunneling. ...
Molecular self-assembly is a well-known technique to create highly functional nanostructures on surfaces. Self-assembly on two-dimensional materials is a developing field and has already resulted in the discovery of several rich and interesting phenomena. Here, we review this progress with an emphasis on the electronic properties of the adsorbates and the substrate in well-defined systems, as unveiled by scanning tunneling microscopy (STM). We cover three aspects of the self-assembly. The first one focuses on non-covalent self-assembly dealing with site-selectivity due to inherent moire pattern present on 2D materials deposited on substrates. Modification of intermolecular interactions and molecule-substrate interactions influences the assembly drastically and 2D materials can also be used as a platform to carry out covalent and metal-coordinated assembly. The second part deals with the electronic properties of molecules adsorbed on 2D materials. By virtue of being inert and possessing low density of states near the Fermi level, 2D materials decouple molecules electronically from the underlying metal substrate and allow high-resolution spectroscopy and imaging of molecular orbitals. The moire pattern on the 2D materials causes site-selective gating and charging of molecules in some cases. The last section covers the effects of self-assembled organic molecules on the electronic properties of graphene as revealed by spectroscopy and electrical transport measurements. Non-covalent functionalization of 2D materials has already been applied for their application as catalysts and sensors. With the current surge of activity on building van der Waals heterostructures from 2D materials, molecular self-assembly has the potential to add an extra level of flexibility and functionality for applications ranging from flexible electronics and OLEDs to novel electronic devices and spintronics.
... The XTB-calculated energies for c-P40 nanorings indicate a linear dispersion relationship for states around the HOMO and LUMO (i.e., equally spaced in energy). This is in agreement with dI=dV measurements for linear oligothiophene molecular systems [41], although in contrast to that observed for hexagonal structures formed from metal adatoms [11] and small molecular rings [12]. The origin of the apparent discrepancy may be due to the interaction with the metal substrate, with the c-P40 ring stacks and oligothiophene [supported on a NaCl=Cuð111Þ surface [41] ] both likely to be at least partially decoupled from the metallic surface states. ...
... This is in agreement with dI=dV measurements for linear oligothiophene molecular systems [41], although in contrast to that observed for hexagonal structures formed from metal adatoms [11] and small molecular rings [12]. The origin of the apparent discrepancy may be due to the interaction with the metal substrate, with the c-P40 ring stacks and oligothiophene [supported on a NaCl=Cuð111Þ surface [41] ] both likely to be at least partially decoupled from the metallic surface states. ...
The electronic structure of a molecular quantum ring (stacks of 40-unit cyclic porphyrin polymers) is characterized via scanning tunneling microscopy and scanning tunneling spectroscopy. Our measurements access the energetic and spatial distribution of the electronic states and, utilizing a combination of density functional theory and tight-binding calculations, we interpret the experimentally obtained electronic structure in terms of coherent quantum states confined around the circumference of the π-conjugated macrocycle. These findings demonstrate that large (53 nm circumference) cyclic porphyrin polymers have the potential to act as molecular quantum rings.
... We proceed in the framework of a generalized master equation, which naturally allows for the treatment of strong electronic correlations on the molecule [8,[50][51][52]. The latter plays a crucial role in STM on thin insulating film, as the presence of the insulator hinders the screening associated to the hybridization with the metallic substrate and enhances the specific features of the pristine molecule [5,7,8,53]. The starting point is the Liouville-von Neumann (LvN) equation for the full density matriẋ ...
... In support of our argument we notice, moreover, that the differential conductance peaks in STM experiments on thin insulating films [5,7,53,56] show a gaussian profile and a width clearly much larger than both the thermal energy k B T and the tunnelling induced broad-ening Γ. The role of substrate optical phonons in the explanation of such anomalous spectral broadening of the conductance has been demonstrated [57] and also controlled by changing the insulating layer from a NaCl to a RbI or Xe one [56]. ...
Recent lightwave-STM experiments have shown space and time resolution of single molecule vibrations directly on their intrinsic length and time scales. We address here theoretically the electronic dynamics of a copper-phthalocynanine in a lightwave-STM, explored within a pump-probe cycle scheme. The spin-orbit interaction in the metallic center induces beatings of the electric charge flowing through the molecule as a function of the delay time between the pump and the probe pulses. Interference between the quasi-degenerate anionic states of the molecule and the intertwined dynamics of the associated spin and pseudospin degrees of freedom are the key aspects of such phenomenon. We study the dynamics directly in the time domain within a generalized master equation approach.
... The individual satellites are broadened (due to the coupling with the substrate) and cannot be resolved; their sum results in the overall lineshape. [37,41] The dI/dV spectra recorded on DCA 3 Au 2 network show a delocalized peak at 1.2 V that is present all over the intact network. However, if the Au coordination structure is broken, this state is locally suppressed over the missing Au atom (not shown). ...
... This is further evidenced by the dI/dV map at 1.6 V. If the other component at 1.6 V was simply due to phonon replicas, similar to the close-packed DCA islands, we would expect no differences in the spatial distribution of the dI/dV 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 signal at 1.2 V and 1.6 V. [41,44,45] Instead, there is significantly more intensity on the DCA molecules. These findings indicate that the resonance in the dI/dV spectrum indeed corresponds to contributions of both Au atom states and the molecular orbitals of DCA. ...
On‐surface metal‐organic coordination provides a promising way for synthesizing different two‐dimensional lattice structures that have been predicted to possess exotic electronic properties. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we studied the supramolecular self‐assembly of 9,10‐dicyanoanthracene (DCA) molecules on the Au(111) surface. Close‐packed islands of DCA molecules and Au‐DCA metal‐organic coordination structures coexist on the Au(111) surface. Ordered DCA3Au2 metal‐organic networks have a structure combining a honeycomb lattice of Au atoms with a kagome lattice of DCA molecules. Low‐temperature STS experiments demonstrate the presence of a delocalized electronic state containing contributions from both the gold atom states and the lowest unoccupied molecular orbital of the DCA molecules. These findings are important for the future search of topological phases in metal‐organic networks combining honeycomb and kagome lattices with strong spin‐orbit coupling in heavy metal atoms.
... De façon générale, en spectroscopie tunnel, la résolution spatiale de la pointe ne permet pas d'accéder aux composantes hautes fréquences de cesétats mais uniquement aux modulations basses fréquences. Dans de nombreux exemples de la littérature, cesétats LUMOs sont donc perçus simplement comme les premiers modes de vibration d'unétat confinéà une dimension [91,92]. C'estégalement de cette manière qu'ils seront décrits dans la suite de ce chapitre. ...
... et coll. en 2010[91]. ...
Dans ce travail nous démontrons, au travers de deux études, l'intérêt fondamental du couplage des techniques de photoémission résolue en angle (ARPES) et de spectroscopie tunnel (STS) dans l'analyse des propriétés électroniques d'interfaces nanostructurées. Dans la première partie, nous présentons une méthodologie permettant de déduire le potentiel de surface induit par la reconstruction triangulaire d'une monocouche d'Ag/Cu(111). Cette méthode est basée sur la mesure des gaps caractérisant la structure de bande de l'état de Shockley du système aux points de haute symétrie de la zone de Brillouin. L'évaporation d'adatomes de potassium permet d'augmenter le nombre de gaps accessibles en photoémission en décalant les bandes vers les états occupés. Dans un modèle d'électrons presque libres, leur amplitude nous donne accès aux premières composantes de Fourier du potentiel. La reconstruction de ce dernier dans l'espace direct nous permet ensuite de calculer la densité d'états locale que nous comparons aux mesures de conductance STS. La seconde partie est consacrée à l'étude de la croissance et des propriétés électroniques des molécules de 1,4-dibromobenzène (DBB) et 1,4-diiodobenzène (DIB) évaporées sur Cu(110). Leur dépôt à température ambiante sur la surface entraîne la déshalogénation des molécules et la formation de phases organométalliques. A 200°C, le système polymérise pour former des chaînes unidimensionnelles de poly(p-phénylène) parfaitement alignées. Les mesures ARPES révèlent l'existence d'une bande pi unidimensionnelle d'états HOMOs dispersant sous le niveau de Fermi. En STS, nous observons également, pour des petites chaînes, le confinement des états LUMOs dans la partie inoccupée du spectre. Le déconfinement de ces états pour les grandes chaînes conduit à la formation d'une bande continue croisant le niveau de Fermi, conférant au polymère un caractère métallique 1D. Le gap HOMO-LUMO est alors mesuré à 1.15 eV
... Single-molecule junctions have gained a lot of interest over the past few decades 1-3 where many interesting phenomena have been found, such as Coulomb blockades, [4][5][6][7] Kondo effects, [8][9][10][11][12] and Franck-Condon blockades. [13][14][15][16] It is now well known that electron-nuclear couplings can play an important role in many molecular junction transport processes, 17,18 leading to heating, [19][20][21][22][23] nonadiabatic effects, [24][25][26][27] enhanced current fluctuations, [28][29][30] hysteresis or switching, [31][32][33][34] negative differential resistance, [35][36][37][38][39] and current induced chemistry. [40][41][42][43][44] To understand these phenomena, theoretical insight can be gained from a non-equilibrium Green's function (NEGF) [45][46][47][48][49][50][51][52][53][54][55] formalism, the quantum master equation (QME), 39,45,[56][57][58] and semiclassical methods. ...
... We next express the bQME [Eq. (26)] in the respective eigenbases ofĤ 0 andĤ 1 , ...
We extend the broadened classical master equation (bCME) approach [W. Dou and J. E. Subotnik, J. Chem. Phys. 144, 024116 (2016)] to the case of two electrodes, such that we may now calculate non-equilibrium transport properties when molecules come near metal surfaces and there is both strong electron-nuclear and strong metal-molecule coupling. By comparing against a numerically exact solution, we show that the bCME usually works very well, provided that the temperature is high enough that a classical treatment of nuclear motion is valid. Finally, in the low temperature (quantum) regime, we suggest a means to incorporate broadening effects in the quantum master equation (QME). This bQME works well for fairly low temperatures.
... Despite the high lateral resolution of a STM, the short lifetime of charge states in molecular species in direct contact with a metal substrate typically results in a broadened peak in the differential conductance, making the detailed observation of vibrational excitations challenging 4,35 . This problem can be circumvented by decoupling molecules from the metal using an insulating layer to better resolve them with high intensity and resolution 22,27,[36][37][38] . However, exploring vibrational excitations in molecular assemblies on decoupling layers is rather scarce in the literature since assembly protocols are less established than those on metals. ...
Electron-vibration coupling is of critical importance for the development of molecular electronics, spintronics, and quantum technologies, as it affects transport properties and spin dynamics. The control over charge-state transitions and subsequent molecular vibrations using scanning tunneling microscopy typically requires the use of a decoupling layer. Here we show the vibronic excitations of tetrabromotetraazapyrene (TBTAP) molecules directly adsorbed on Ag(111) into an orientational glassy phase. The electron-deficient TBTAP is singly-occupied by an electron donated from the substrate, resulting in a spin 1/2 state, which is confirmed by a Kondo resonance. The TBTAP•− discharge is controlled by tip-gating and leads to a series of peaks in scanning tunneling spectroscopy. These occurrences are explained by combining a double-barrier tunneling junction with a Franck-Condon model including molecular vibrational modes. This work demonstrates that suitable precursor design enables gate-dependent vibrational excitations of molecules on a metal, thereby providing a method to investigate electron-vibration coupling in molecular assemblies without a decoupling layer.
... Vibrations correlate with the electronic structure of the molecule 6 and the symmetry of the electronic excitation, 7 they also reveal coherent electronnuclear coupling. 8 Moreover, the mutual influence between the mechanical and the electronic dynamics can range from being a small perturbation to large one. In the latter case non-perturbative effects are visible in the Franck-Condon blockade [9][10][11] with associated electronic avalanche, 12 in the regular shuttle dynamics 13,14 with virtually vanishing shot noise, 15 in run away modes 16,17 which ultimately bring to molecular dissociation. ...
We study the transport properties of an Anderson-Holstein model with orbital degeneracies and a tunneling phase that allows for the formation of dark states. The resulting destructive interference yields a characteristic pattern of positive and negative differential conductance features with enhanced shot noise, without further asymmetry requirements in the coupling to the leads. The transport characteristics are strongly influenced by the Lamb-shift renormalization of the system Hamiltonian. Thus, the electron-vibron coupling cannot be extracted by a simple fit of the current steps to a Poisson distribution. For strong vibronic relaxation, a simpler effective model with analytical solution allows for a better understanding and moreover demonstrates the robustness of the described effects.
... Recent tunneling experiments have revealed some limitations of the Franck-Condon model. In cases where the electronic energy level spacing was similar to vibrational energies, it was found that avoided level crossings determine the resonant sidebands [24,25]. In other cases, intensity variations of the resonant sidebands along an organic molecule were interpreted in terms of coherent vibrational modes with different symmetries [8] or with vibration-assisted coupling of wave functions of different symmetry in molecule and tip [26]. ...
Vibronic spectra of molecules are typically described within the Franck-Condon model. Here, we show that highly resolved vibronic spectra of large organic molecules on a single layer of MoS2 on Au(111) show spatial variations in their intensities, which cannot be captured within this picture. We explain that vibrationally mediated perturbations of the molecular wave functions need to be included into the Franck-Condon model. Our simple model calculations reproduce the experimental spectra at arbitrary position of the scanning tunneling microscope’s tip over the molecule in great detail.
... Recent tunneling experiments have revealed some limitations of the Franck-Condon model. In cases where the electronic energy level spacing was similar to vibrational energies, it was found that avoided level crossings determine the resonant sidebands [24,25]. In other cases, intensity variations of the resonant sidebands along an organic molecule were interpreted in terms of coherent vibrational modes with different symmetries [8] or with vibration-assisted coupling of wave functions of different symmetry in molecule and tip [26]. ...
Vibronic spectra of molecules are typically described within the Franck-Condon model. Here, we show that highly resolved vibronic spectra of large organic molecules on a single layer of MoS$_{2}$ on Au(111) show spatial variations in their intensities, which cannot be captured within this picture. We explain that vibrationally mediated perturbations of the molecular wave functions need to be included into the Franck-Condon model. Our simple model calculations reproduce the experimental spectra at arbitrary position of the STM tip over the molecule in great detail.
... Computational chemistry is not a substitution for experimental studies, but plays a significant role in enabling chemists to clarify and rationalize known chemistry and explore new or unknown chemistry [1]. Recently, many experimental and theoretical efforts have been carried out to characterize the electrical transport properties of molecular nanowires [2,3]. To understand the charge transport mechanism of metal-molecule junctions experimentally is still a challenging one due to several kinds of uncontrollable experimental factors that affect the resulting data [4]. ...
ABSTRACT
The theoretical electronic structure and transport properties of Au and thiol substituted 2,5-diphenylthiophene have
been calculated from high level Density functional theory (DFT) using B3LYP method with LANL2DZ basis set. The
molecular geometric parameters predicted by DFT method are in agreement with the reported results. Variation in
MPA and NPA atomic charges and the electric dipole moment of the molecules of various levels of applied electric
fields have been analyzed. The decrease of HOMO-LUMO gap from 2.12 eV to 0.94 eV determined from density of
states spectrum for the applied field (0 – 0.21 VÅ-1) shows that thiol linked 2,5-diphenylthiophene molecule can act
as efficient molecular nanowire for Au electrodes.
Keywords: Electronic structure, Atomic charges, Molecular orbital analysis, Electric dipole moment
... Similar spectral features are commonly observed in tunneling spectroscopy of molecules [43]. We note that the vibronic features appear in dI=dV, as opposed to d 2 I=dV 2 , because of the double-barrier tunneling junction geometry [44]. We expect that several phonon modes are excited by the transient electron attachment during tunneling. ...
Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping. We report on the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS2 by a combination of CO-tip noncontact atomic force microscopy and scanning tunneling microscopy. Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states followed by vibronic sidebands provide a unique fingerprint of this defect. Direct imaging of the defect orbitals, together with ab initio GW calculations, reveal that the large splitting of 252±4 meV between these defect states is induced by spin-orbit coupling.
... Similar to the setting on ultra-thin halides (e.g., Refs. [155][156][157][158]160,161,831,832]), the STM's capabilities to probe adsorbates in a weak-coupling regime with atomic precision is expected to provide further fascinating insights into molecular properties, functionalities and on-surface chemistry. For many experiments and applications however, the hBN monolayer thickness of course poses a serious limitation: considerably thicker films would be needed to prevent charge tunneling [179,180] and to avoid a (spacer layer thickness dependent) influence of the buried metal on mobility and self-assembly properties [167,168]. ...
Hexagonal boron nitride (hBN) monolayers have attracted considerable interest as atomically thin sp ² -hybridized sheets that are readily synthesized on various metal supports. They complement the library of two-dimensional materials including graphene and open perspectives for van der Waals heterostructures. In this review, we discuss the surface science of hBN including its growth, the hBN/metal interface and its application as template for adsorbates. We mainly focus on experimental studies on hBN/metal single crystals under ultra-high vacuum conditions. The interfaces are classified regarding their geometric structure - ranging from planar to strongly corrugated overlayers - and their electronic properties - covering weakly and strongly interacting systems. The main part of this review deals with hBN/metal substrates acting as supports for adsorbates such as individual atoms, metal clusters, organic molecules, metal-organic complexes and networks. We summarize recent surface science studies that reveal the unique role of the hBN/metal interfaces in tailoring characteristic properties of such adsorbates. Central aspects include templating and self-assembly, catalytic activity and on-surface reactions, electronic and magnetic structure. As many of the resulting systems feature superstructures with periodicities in the nanometer range, a length scale also reflecting the size of adsorbates, scanning probe microscopy is one of the most common techniques employed. In short, the goal of this review is to give an overview on the experimental and complementary theoretical studies on hBN templates available to date and to highlight future perspectives.
... A l'instar du système dBB/Cu(110) ainsi que d'autres systèmes présents dans la littérature [165,206,207], on observe des modulations de la densité électronique associée aux états LUMOs le long des polymères. Ces modulations peuvent être interprétées comme les premiers modes d'une onde électronique confinée à une dimension. ...
Dans ce travail, nous illustrons l’avantage de coupler les techniques de photoémission résolue en angle (ARPES) et de microscopie/spectroscopie tunnel (STM/STS) pour l'étude des propriétés électroniques et structurales des surfaces/interfaces nanostructurées. Dans la première partie, nous présentons l’étude du supraconducteur non conventionnel Eu(Fe0.86Ir0.14)2As2. Ce composé, dopé en Ir de manière optimale, possède une phase supraconductrice réentrante (Tc=22K) qui coexiste avec un ordre ferromagnétique (TM=18K). Nous présentons une étude par ARPES de la structure de bande dans le plan et hors plan ainsi que de la surface de Fermi. Les bandes associées aux états 3d du fer, responsables de la supraconductivité, sont modifiées en présence de la substitution en Ir, mais la topologie de la surface de Fermi est conservée. Le gap supraconducteur est mesuré à 5.5 meV, supérieur à la valeur estimée par la théorie BCS pour une température Tc=22K. La disparition du gap au-dessus de T=10K coïncide avec la phase résistive induite par l’ordre magnétique des moments Eu2+. Les modifications de la surface de Fermi dans le composé substitué indiquent clairement un dopage effectif en trou par rapport au composé parent. La seconde partie est consacrée à l’étude de la croissance, des mécanismes de polymérisation et des conséquences sur les propriétés électroniques de nanostructures moléculaires. Celles-ci sont élaborées par évaporation sous vide des molécules 1,4-dibromobenzène (dBB) et 1,4-diiodobenzène (dIB) sur les surfaces de Cu(110), Cu(111) et Cu(775) en utilisant la réaction catalytique de Ullmann. Nous avons étudié l’influence du type d’halogène et de substrat sur la réaction de polymérisation ainsi que les conséquences sur les propriétés électroniques. En particulier, nous mettons en évidence par des mesures STM et NEXAFS (mesures effectuées à l’aide du rayonnement synchrotron) un mécanisme original de croissance des polymères sur la surface de Cu(775) qui s’accompagne d’une restructuration à l’échelle nanométrique sous la forme d’un « step-bunching ». Celui-ci conduit à la formation de polymères de grande longueur et parfaitement ordonnés à grande échelle. En combinant les mesures ARPES et STS, nous mettons en évidence une évolution du gap HOMO-LUMO caractérisant les chaînes de poly(para)phénylène ainsi formées avec le type d’halogène impliqué dans la réaction catalytique et la géométrie du substrat. Nous montrons ainsi que si le caractère métallique du polymère élaboré sur le Cu(110) trouve son origine dans sa forte interaction avec le substrat, celle-ci diminue fortement lorsque la synthèse a lieu sur les surfaces de Cu(111) et de Cu(775) conduisant à retrouver un comportement semi-conducteur caractérisé par un gap HOMO-LUMO évalué à 2.2 eV
... [114] Mit der STM lassen sich in der Regel HOMO und LUMO bildlich auflçsen, aber das Spannungsfenster,b ei dem ein Tu nnelstrom mçglich ist, ist auf wenige Volt um das Fermi-Niveau herum begrenzt. Manchmal kçnnen auch einige wenige Molekülorbitale mit Energien unterhalb des HOMO [120] oder oberhalb des LUMO [121] beobachtet werden. Fürd ie Bildgebung von Orbitalen sollte man (aber nicht notwendigerweise) [120] das Moleküle lektronisch durch einen dünnen Isolierfilm wie beispielsweise einer Doppelschicht aus NaCl [108,122] oder einer Monoschicht von Xe-Atomen vom metallischen Substrat abgrenzen. ...
Mit der Rastersondenmikroskopie kann man auf einer Oberfläche adsorbierte Moleküle individuell und mit hoher Auflösung untersuchen. Die Technik, die bei tiefen Temperaturen und im Ultrahochvakuum durchgeführt wird, liefert Informationen über Struktur, Konfiguration, Ladungszustand, Aromatizität und die Beteiligung von Resonanzstrukturen. Durch Funktionalisierung der Spitze eines Rasterkraftmikroskops mit einem CO-Molekül können Einzelmoleküle mit atomarer Auflösung abgebildet werden, ihre Adsorptionsgeometrie kann gemessen und die Bindungsordnungen im Molekül können bestimmt werden. Mit der Rastertunnelmikroskopie und der Kelvinsondenkraftmikroskopie kann man zudem die Elektronendichte der molekularen Grenzorbitale bzw. die Ladungsverteilung innerhalb des Moleküls kartieren. In Kombination ergeben diese Methoden ein hochauflösendes Verfahren zur Identifizierung und Charakterisierung von Einzelmolekülen. Die Empfindlichkeit auf Einzelmoleküle sowie die Option, durch Manipulation von Atomen mit der Spitze des Mikroskops chemische Reaktionen auszulösen, eröffnen einzigartige chemische Anwendungsmöglichkeiten und heben die Rastersondenmikroskopie von den klassischen Methoden zur molekularen Strukturaufklärung heraus. Somit kann die Rasterkraftmikroskopie nicht nur bei der Identifizierung von schwer zu bestimmenden Naturstoffen helfen, sondern sie ist auch ein leistungsstarkes und besonders gut geeignetes Instrument, um Oberflächenreaktionen zu untersuchen und Radikale und molekulare Mischungen zu charakterisieren. In diesem Aufsatz zeigen wir auf, welche Entwicklung die hochauflösende Rastersondenmikroskopie mit funktionalisierter Spitze bei der molekularen Strukturaufklärung und -charakterisierung in den vergangenen Jahren genommen hat, und erörtern die Herausforderungen der kommenden Jahre.
... [114] Typically, HOMO and LUMO densities can be resolved using STM, but the accessible voltage window for tunneling is limited to a few volts around the Fermi level. Sometimes also a few molecular orbitals with energies below the HOMO [120] or above the LUMO [121] can be imaged. For orbital imaging, it is beneficial (but not mandatory [120] ) to decouple the molecule electronically from the metallic substrate using a thin insulating film, for example, a bilayer of NaCl [108,122] or a monolayer of Xe atoms. ...
Using scanning probe microscopy techniques, at low temperatures and in ultrahigh vacuum, individual molecules adsorbed on surfaces can be probed with ultrahigh resolution to determine their structure and details of their conformation, configuration, charge states, aromaticity, and the contributions of resonance structures. Functionalizing the tip of an atomic force microscope with a CO molecule enabled atomic-resolution imaging of single molecules, and measurement of their adsorption geometry and bond-order relations. In addition, by using scanning tunneling microscopy and Kelvin probe force microscopy, the density of the molecular frontier orbitals and the electric charge distribution within molecules can be mapped. Combining these techniques yields a high-resolution tool for the identification and characterization of individual molecules. The single-molecule sensitivity and the possibility of atom manipulation to induce chemical reactions with the tip of the microscope open up unique applications in chemistry, and differentiate scanning probe microscopy from conventional methods for molecular structure elucidation. Besides being an aid for challenging cases in natural product identification, atomic force microscopy has been shown to be a powerful tool for the investigation of on-surface reactions and the characterization of radicals and molecular mixtures. Herein we review the progress that high-resolution scanning probe microscopy with functionalized tips has made for molecular structure identification and characterization, and discuss the challenges it will face in the years to come.
... The dI/dV spectra recorded on DCA of the complex shows that the LUMO shifts down to 750 mV and three satellite vibronic peaks also become visible. The vibronic mode energy of ∼200 mV fits well with the expected energy of the C-C vibration [34,35]. ...
Metal-organic frameworks (MOFs) are an important class of materials that present intriguing opportunities in the fields of sensing, gas storage, catalysis, and optoelectronics. Very recently, two-dimensional (2D) MOFs have been proposed as a flexible material platform for realizing exotic quantum phases including topological and anomalous quantum Hall insulators. Experimentally, direct synthesis of 2D MOFs has been essentially confined to metal substrates, where the interaction with the substrate masks the intrinsic electronic properties of the MOF. Here, we demonstrate synthesis of 2D honeycomb metal-organic frameworks on a weakly interacting epitaxial graphene substrate. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by density-functional theory (DFT) calculations, we show the formation of 2D band structure in the MOF decoupled from the substrate. These results open the experimental path towards MOF-based designer quantum materials with complex, engineered electronic structures.
... Electrons confined to structures with dimensions comparable to their de Broglie wavelength exhibit quantization, which is a fundamental aspect of quantum systems. Striking examples are whispering gallery modes in oligothiophene nanorings [1] and in graphene [2] and linear molecules acting as one-dimensional resonators confining nearly free electrons [3,4]. Electron confinement finds application in technological devices including high-brightness light-emitting diodes, semiconductor lasers, photovoltaics, and spintronics [5][6][7]. ...
We have studied electron states present at the Pb(111) surface above Ar-filled nanocavities created by ion beam irradiation and annealing. Vertical confinement between the parallel crystal and nanocavity surfaces creates a series of quantum well state subbands. Differential conductance data measured by scanning tunneling spectroscopy contain a characteristic spectroscopic fine structure within the highest occupied subband, revealing additional quantization. Unexpectedly, reflection at the open boundary where the thin Pb film recovers its bulk thickness gives rise to the lateral confinement of electrons.
... Some of the key advantages of using oligothiophenes as organic semiconductors are their remarkable stability and the property of being easily functioned by substituting H atoms in the thiophene rings with properly designed chemical groups, which may "tailor" their electronic and optical properties to analyzed one's specific needs [5,6]. Besides those practical advantages; a-oligothiophenes (nT); n being the number of thiophene rings comprised between 3 and 8, offer a suitable set of linearly growing conjugated chains to models optoelectronic properties of polymers [7,8]. Generally, oligothiophenes are processed via relatively cumbersome vapor deposition techniques [9,10] and many experimental subjects had demonstrated the substantial impact of the sound structure of the organic thin films along the device performances [11]. ...
Many experimental studies have established the substantial impact of the sound structure of the organic thin films on optoelectronic device performances. This study is a collection of experimental and theoretical investigation of the radiative recombination kinetics in an organic light emitting diode based on spin cast α-4T. The structural properties of the organic thin films (α-4T) deposited by the spin coating technique are discussed. The electrical characterization of the diode-like samples (ITO/α-4T/Al) have revealed a very low effective mobility of charge carriers in the spin cast α-4Tand the radiative recombination current was extracted. The luminescence-current characterization of the diode-like samples (ITO/α-4T/Al) was investigated and modeled.
... However, transition A3 of CoPC −1 , which would be expected to correspond to N3 of CoPC 0 , should then appear well below -1.0 V. Instead, there are two transitions A3 and A4 just below A2 at energies of ∼-0.3 and ∼-0.8 V, respectively. Note that all the transitions show satellite peaks due to electron-vibration coupling [46,[51][52][53][54]. In addition, maps of the spatial variation of the dI/dV signal shown in the second and fifth row of Fig. 4 reveal that the wave functions corresponding to transitions N3 and A3 exhibit a different shape and symmetry. ...
Scanning tunneling spectroscopy measures how a single electron with definite energy propagates between a sample surface and the tip of a scanning tunneling microscope. In the simplest description, the differential conductance measured is interpreted as the local density of states of the sample at the tip position. This picture, however, is insufficient in some cases, since especially smaller molecules weakly coupled with the substrate tend to have strong Coulomb interactions when an electron is inserted or removed at the molecule. We present theoretical approaches to go from the non-interacting and single-particle picture to the correlated many-body regime. The methodology is used to understand recent experiments on finite armchair graphene nanoribbons and phthalocyanines. We also theoretically discuss the strongly-correlated model system of fractional quantum Hall droplets.
... 26 STM also enables investigations of the effect of molecular conformational disorder on electronic structure by using scanning tunneling spectroscopy (STS), which has been used to study oligothiophene electronic structure on a variety of surfaces. 19,20,[34][35][36][37][38] In particular, STS studies of unsubstituted linear and cyclic oligothiophenes showed that, to a first approximation, electronic states in both types of oligothiophenes can be described as particle-in-a-box (PIAB) states subjected to appropriate boundary conditions. 35 STS studies of substituted oligothiophenes have been less common, 19,20 and no structure-specific analysis were reported, despite the importance of such molecules for solutionprocessable device applications, where substitutional groups are used to enhance oligothiophene solubility. ...
We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of the electronic structures of different alkyl-substituted oligothiophenes on the Au(111) surface.
STM imaging showed that on Au(111), oligothiophenes adopted distinct straight and bent conformations. By combining STS maps with STM images, we visualize, in real space, particle-in-a-box-like oligothiophene molecular orbitals. We demonstrate that different planar conformers with significant geometrical distortions of oligothiophene backbones surprisingly exhibit very similar electronic structures, indicating a low degree of conformation-induced electronic disorder. The agreement of these results with gas-phase density functional theory calculations implies that the oligothiophene interaction with the Au(111) surface is generally insensitive to molecular conformation.
... Local spectroscopy and tight-binding modelling. Using STS, confinement of unoccupied molecular states has been observed previously in individual polythiophene chains 44 , and more recently, in 7-AGNRs 35 and GNR heterojunctions 45 . We recorded differential conductivity maps as a function of chain length, to build the k-resolved band structure associated with the conduction band of the infinite polymer. ...
On-surface covalent self-assembly of organic molecules is a very promising bottom–up approach for producing atomically controlled nanostructures. Due to their highly tuneable properties, these structures may be used as building blocks in electronic carbon-based molecular devices. Following this idea, here we report on the electronic structure of an ordered array of poly(para-phenylene) nanowires produced by surface-catalysed dehalo- genative reaction. By scanning tunnelling spectroscopy we follow the quantization of unoc- cupied molecular states as a function of oligomer length, with Fermi level crossing observed for long chains. Angle-resolved photoelectron spectroscopy reveals a quasi-1D valence band as well as a direct gap of 1.15 eV, as the conduction band is partially filled through adsorption on the surface. Tight-binding modelling and ab initio density functional theory calculations lead to a full description of the band structure, including the gap size and charge transfer mechanisms, highlighting a strong substrate–molecule interaction that drives the system into a metallic behaviour.
Although the efficient separation of electron-hole (e-h) is one of the most sought-after electronic characteristics in materials, due to thermally induced atomic motion and other factors, they do not remain...
Understanding the local impact of environmental electronic perturbations on the local density of states (LDOS) of single-walled carbon nanotubes (CNTs) is critical for developing CNT-based devices. We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of CNTs adsorbed on both a metal, Au(111), and a dielectric, monolayer RbI on Au(111), serving as models for stronger and weaker electrostatic interactions, respectively. In both cases, STS revealed modulations in the CNT LDOS corresponding to features in the underlying material. We corroborate our STM/STS results with density functional theory calculations of the electronic structure of semiconducting CNTs in the presence and absence of an external dipole. DFT-calculated CNT LDOS qualitatively matched STM/STS results, providing key insight into the local impact external charges have on CNT electronic structure.
The interaction between electronic and vibrational degrees of freedom is an important mechanism in nonequilibrium charge transport through molecular nanojunctions. While adiabatic polaron-type coupling has been studied in great detail, new transport phenomena arise for nonadiabatic coupling scenarios corresponding to a breakdown of the Born-Oppenheimer approximation. Employing the numerically exact hierarchical equations of motion approach, we analyze the effect of nonadiabatic electronic-vibrational coupling on electron transport in molecular junctions considering a series of models with increasing complexity. The results reveal a significant influence of nonadiabatic coupling on the transport characteristics and a variety of interesting effects, including negative differential conductance. The underlying mechanisms are analyzed in detail.
Recent lightwave-STM experiments have shown space and time resolution of single molecule vibrations directly on their intrinsic length and timescales. We address here theoretically the electronic dynamics of a copper-phthalocyanine in a lightwave-STM explored within a pump-probe cycle scheme. The spin-orbit interaction in the metallic center induces beatings of the electric charge flowing through the molecule as a function of the delay time between the pump and the probe pulses. Interference between the quasidegenerate anionic states of the molecule and the intertwined dynamics of the associated spin and pseudospin degrees of freedom are the key aspects of such phenomenon. We study the dynamics directly in the time domain within a generalized master-equation approach.
We study the transport properties of an Anderson-Holstein model with orbital degeneracies and a tunneling phase that allows for the formation of dark states. The resulting destructive interference yields a characteristic pattern of positive and negative differential conductance features with enhanced shot noise, without further asymmetry requirements in the coupling to the leads. The transport characteristics are strongly influenced by the Lamb-shift renormalization of the system Hamiltonian. Thus, the electron-vibron coupling cannot be extracted by a simple fit of the current steps to a Poisson distribution. For strong vibronic relaxation, a simpler effective model with analytical solution allows for a better understanding and moreover demonstrates the robustness of the described effects.
This article highlights the important role of scanning tunneling and atomic force microscopy in modern surface science experiments. Imaging with atomic resolution, manipulation of matter atom by atom, spectroscopy of confined electrons, molecular vibrational quanta, surface phonons, single‐atom spin flips, and single‐molecule fluorescence photons are some of the diverse applications of the microscopes. The impact of the actual atomic or molecular termination of the tip is emphasized. A variety of examples presents the state of the art in quantum physics of surfaces and interfaces and demonstrates that scanning probe techniques significantly contribute to the understanding of matter at the atomic scale.
The modern history of organic electronics based on conducting polymers started with doped polyacetylene in 1970’s. However, polyacetylene suffers many problems, such as structural disorder along C-C single bonds, the resulting short effective conjugation length, and insolubility. To address these issues, many efforts were made in terms of partial rigidification of the polyene structure with heteroatom and carbon linkages. Among them, oligo(phenylenevinylene)s (OPVs) are all-carbon analogues of polyacetylenes, albeit many C-C single bonds that can freely rotate are still left in a molecular framework. We envisioned that full linkages between each phenylene and vinylene unit using sp³-carbon atoms can rigidify the entire OPV skeleton. Indeed, methylene-bridged stilbene was prepared in 1922, and the longer homologue of such carbon-bridged oligo(phenylenevinylene)s (COPV) is a framework of which construction had been a long-term challenge in a field of synthetic organic chemistry. In 2009 we have reported the synthesis of COPV based on a novel intramolecular cyclization reaction to afford a dilithiated indacene framework, a key intermediate to construct the COPV framework. Thus prepared COPV were found to show not only excellent photophysical and electronic properties due to the rigid planar π-conjugated framework, but also high stability and solubility due to the organic side chains installed on the bridging carbon atoms that sterically protect the π-conjugated framework. With these features, the COPV molecules have also served as versatile materials at a single-molecular and a bulk level, such as in organic thin-film lasers, dye-sensitized and perovskite solar cells, and molecular wires. Remarkable discoveries in the area connecting chemistry and physics include inelastic tunneling and long-range resonance tunneling at ambient temperature, which were previously observed only for organic molecules under cryogenic conditions. This class of newly prepared molecules created by the power of organic synthesis will serve as versatile materials for fundamental and applied researches in a broad range of field.
We investigate the nonequilibrium spin transport through an electron-phonon coupled quantum dot, on which an external magnetic field B0 and a rotating magnetic field [B1cos(ω1t),B1sin(ω1t)] are applied. It is found that the spin current is significantly affected by the destructive interference between electron tunneling waves through different spin channels. As a result, dips appear in the spin current as a function of ω1 every time the Rabi frequency is tuned to be integral numbers of the phonon frequency. It is also found that the main spin current peak does not always exist at the resonant rotating frequency ω1=gμBB0.
The modern history of conducting organic systems started with a fortuitous error in 1967 on acetylene polymerization, followed by a rational discovery in 1976 on the effects of doping that generates a polaron and, hence, dramatically increases conductivity. Not unexpectedly, however, the prototypical polyacetylene suffers many problems, including C–C single bond rotation, short effective conjugation length, radiationless deactivation, and instability of the polarons. Several strategies have been put in place to solve these problems. An early approach relied on partial rigidification of the polyene structure by conversion into polymers with thiophene, pyrrole, and benzene linkages. An oligo(phenylene vinylene) (OPV) is an all-carbon analogue of polyacetylene, where every other diene unit in the polyene chain is converted to a benzene unit, still leaving many C–C single bonds freely rotating in the molecule. We considered adding additional carbon bridges to rigidify the OPV skeleton entirely to create a carbon-bridged OPV (COPV).
We study the electron tunneling through a single level quantum dot in the presence of electron–phonon interaction. By using the Green’s function and canonical transformation methods, we calculated exactly the current. It is found that the current vs dot level exhibits satellite peaks even without occurring of phonon-assisted tunneling processes, and properties of the current are affected heavily by the strength of electron–phonon interaction and phonon temperature.
We introduce an auxiliary quantum master equation dual fermion method and argue that it presents a convenient way to describe steady states of correlated impurity models. The scheme yields an expansion around a reference that is much closer to the true nonequilibrium state than that in the original dual fermion formulation. In steady-state situations, the scheme is numerically inexpensive and avoids time propagation. The Anderson impurity model is used to test the approach against numerically exact benchmarks.
Charge transport through molecular junctions is often described either as a purely coherent or a purely classical phenomenon, and described using the Landauer-Büttiker formalism or Marcus theory (MT), respectively. Using a generalised quantum master equation, we here derive an expression for current through a molecular junction modelled as a single electronic level coupled with a collection of thermalised vibrational modes. We demonstrate that the aforementioned theoretical approaches can be viewed as two limiting cases of this more general expression and present a series of approximations of this result valid at higher temperatures. We find that MT is often insufficient in describing the molecular charge transport characteristics and gives rise to a number of artefacts, especially at lower temperatures. Alternative expressions, retaining its mathematical simplicity, but rectifying those shortcomings, are suggested. In particular, we show how lifetime broadening can be consistently incorporated into MT, and we derive a low-temperature correction to the semi-classical Marcus hopping rates. Our results are applied to examples building on phenomenological as well as microscopically motivated electron-vibrational coupling. We expect them to be particularly useful in experimental studies of charge transport through single-molecule junctions as well as self-assembled monolayers.
We present a novel hierarchical quantum master equation (HQME) approach which provides a numerically exact description of nonequilibrium charge transport in nanosystems with electronic-vibrational coupling. In contrast to previous work [Phys. Rev. B $\bf{94}$, 201407 (2016)], the active vibrational degrees of freedom are treated in the reservoir subspace and are integrated out. This facilitates applications to systems with very high excitation levels, for example due to current-induced heating, while properties of the vibrational degrees of freedom, such as the excitation level and other moments of the vibrational distribution function, are still accessible. The method is applied to a generic model of a nanosystem, which comprises a single electronic level that is coupled to fermionic leads and a vibrational degree of freedom. Converged results are obtained in a broad spectrum of parameters, ranging from the nonadiabatic to the adiabatic transport regime. We specifically investigate the phenomenon of vibrational instability, that is, the increase of current-induced vibrational excitation for decreasing electronic-vibrational coupling. The novel HQME approach allows us to analyze the influence of level broadening due to both molecule-lead coupling and thermal effects. Results obtained for the first two moments suggest that the vibrational excitation is always described by a geometric distribution in the weak electronic-vibrational coupling limit.
Using a scanning tunneling microscope, confined electron states are studied that exist above subsurface nanometer-sized voids at Pb(111), where potential barriers at the parallel vacuum-Pb(111) and Pb(111)-void interfaces establish a principal series of quantum well states that are further confined laterally by strong reflection at the open boundaries at the edges of the void. The influence of the size, depth, and shape of the voids on the effectiveness of the lateral confinement is discussed. Standing wave patterns observed in differential conductance maps unravel the dispersion of the relevant underlying Pb electron states.
Graphene quantum dots (GQDs) hold great promise for applications in electronics, optoelectronics and bioelectronics, but the fabrication of widely tunable GQDs has remained elusive. Here, we report the fabrication of atomically precise GQDs consisting of low-bandgap N = 14 armchair graphene nanoribbon (AGNR) segments that are achieved through edge fusion of N = 7 AGNRs. The so-formed intraribbon GQDs reveal deterministically defined, atomically sharp interfaces between wide and narrow AGNR segments and host a pair of low-lying interface states. Scanning tunneling microscopy/spectroscopy measurements complemented by extensive simulations reveal that their energy splitting depends exponentially on the length of the central narrow bandgap segment. This allows tuning of the fundamental gap of the GQDs over one order of magnitude within a few nanometers length range. These results are expected to pave the way for the development of widely tunable intraribbon GQD-based devices.
π-conjugated organic molecules tend to adsorb in a planar configuration on graphene irrespective of their charge state. In contrast, here we demonstrate charging induced strong structural relaxation of tetrafluoro-tetracyanoquinodimethane (F4TCNQ) on epitaxial graphene (G) on Ir(111). The work function modulation over the graphene moiré unit cell causes site-selective charging of F4TCNQ. Upon charging, the molecule anchors to the fcc sites of the G/Ir(111) moiré through one or two cyano groups. The reaction is reversible and can be triggered on a single molecule by moving it between different adsorption sites. We introduce a model taking into account the trade-off between tilt-induced charging and reduced vdW interactions, which provides a general framework for understanding charging-induced structural relaxation on weakly interacting substrates. In addition, we argue that the partial sp(3) rehybridization of the underlying graphene and the possible bonding mechanism between the cyano-groups and the graphene substrate is also relevant for the complete understanding of the experiments. These results provide insight into molecular charging on graphene and they are directly relevant for potential device applications where the use of molecules has been suggested for doping and band structure engineering.
The frontier orbital sequence of individual dicyanovinyl-substituted oligothiophene molecules is studied by means of scanning tunneling microscopy. On NaCl/Cu(111) the molecules are neutral and the two lowest unoccupied molecular states are observed in the expected order of increasing energy. On NaCl/Cu(311), where the molecules are negatively charged, the sequence of two observed molecular orbitals is reversed, such that the one with one more nodal plane appears lower in energy. These experimental results, in open contradiction with a single-particle interpretation, are explained by a many-body theory predicting a strongly entangled doubly charged ground state.
The drive toward miniaturization of electronic devices motivates investigations of atomic structures at semiconductor surfaces. In this chapter, we describe a full protocol of formation of atomic wires on Ge(001):H-(2×1) surface. The wires are composed of bare germanium dimers possessing dangling bonds, which introduce electronic states within the Ge(001):H surface band gap. With a view to the possible applications, we present detailed analysis of the electronic properties of short DB dimer lines and discuss strong electron–phonon coupling observed in STM experiments on single DB dimers. For longer DB dimer wires, this coupling is attenuated making their usage in future nanoelectronic devices feasible.
A new series of mesitylene-linked oligothiophenes (nT, n is the number of thiophene units) including 2T-M, 3T-M, 4T-M, 4T-M-2T, and 4T-M-3T were prepared to investigate the intramolecular hole transfer (HT) from excited radical cation for the first time. The results of spectroscopic and theoretical studies indicated that mesitylene acts as a spacer minimizing the perturbation to the thiophene π-conjugation and increasing the stability of nT radical cations (nT•+). Femtosecond laser flash photolysis was applied to the FeCl3-oxidized 4T•+-M, 4T•+-M-2T, and 4T•+-M-3T. Upon 670 nm laser excitation, the transient absorption spectra of 4T•+-M showed the existence of two species as the D1 and D0hot states. The intramolecular HT processes from excited 4T•+ with the time constants of 1.6 and 0.8 ps were observed upon excitation of 4T•+-M-2T and 4T•+-M-3T, respectively. This is the first capture of such ultrafast processes with the subsequent back HT from the ground-state 2T•+ or 3T•+ in nT assemblies. The current findings indicated an accelerated migration of photocarriers (polarons) in thiophene-based p-type semiconductor materials upon irradiation and provided a fresh viewpoint to understand the successive HT in polythiophenes for various organic molecular devices.
We introduce diagrammatic technique for Hubbard nonequilibrium Green functions (NEGF). The formulation is an extension of equilibrium considerations for strongly correlated lattice models to description of current carrying molecular junctions. Within the technique intra-system interactions are taken into account exactly, while molecular coupling to contacts is used as a small parameter in perturbative expansion. We demonstrate the viability of the approach with numerical simulations for a generic junction model of quantum dot coupled to two electron reservoirs.
Three oligothiophene (terthiophene, tetrathiophene and pentathiophene) derivatives are synthesized and their monolayer self-assemblies on gold (Au) are prepared via Au-S covalent bond. Our UV-Vis experimental characterization of solution reveals the dependence of the optical properties on the conjugation length of the oligothiophenes, which compares well with Time-Dependent Density Functional Theory (TDDFT) simulations of spectra of individual chromophores. Photoluminescent spectra of thin films show pronounced red shifts compared to that of solutions, suggesting strong inter-oligomer interactions. The comparative studies of cyclic voltammograms of tetrathiophene from solution, cast film and self-assembled monolayer (SAM) indicate presence of one, two, and three oxidized species in these samples, respectively, suggesting a very strong electronic coupling between tetrathiophene molecules in the SAM. Scanning tunneling microscopy (STM) imaging of SAMs of the tetrathiophene on an atomically flat Au surface exhibits formation of monolayer assemblies with molecular order, and the molecular packing appears to show an overlay of oligothiophene molecules on top of another one. In contrast, the trimer and pentamer images show only aggregated species lacking long-range order on the molecular level. Such trends in going from disordered-ordered-disordered monolayer assemblies are mainly due to a delicate balance between inter-chromophore π- π couplings, hydrophobic interaction and the propensity to form Au-S covalent bond. Such hypothesis has been validated by our computational results suggesting different interaction patterns of oligothiophenes with odd numbered and even numbered thiophene repeat units placed in a dimer configuration. Observed correlations between oligomer geometry and structural order of monolayer assembly elucidate important structure-property relationships and have implications for these molecular structures in organic optoelectronic devices and energy devices.
Surface-confined dehalogenation reactions are versatile bottom-up approaches for the synthesis of carbon-based nanostructures with predefined chemical properties. However, for devices generally requiring low-conductivity substrates, potential applications are so far severely hampered by the necessity of a metallic surface to catalyze the reactions. In this work we report the synthesis of ordered arrays of poly(p-phenylene) chains on the surface of semiconducting TiO2(110) via a dehalogenative homocoupling of 4,4″-dibromoterphenyl precursors. The supramolecular phase is clearly distinguished from the polymeric one using low-energy electron diffraction and scanning tunneling microscopy as the substrate temperature used for deposition is varied. X-ray photoelectron spectroscopy of C 1s and Br 3d core levels traces the temperature of the onset of dehalogenation to around 475 K. Moreover, angle-resolved photoemission spectroscopy and tight-binding calculations identify a highly dispersive band characteristic of a substantial overlap between the precursor's π states along the polymer, considered as the fingerprint of a successful polymerization. Thus, these results establish the first spectroscopic evidence that atomically precise carbon-based nanostructures can readily be synthesized on top of a transition-metal oxide surface, opening the prospect for the bottom-up production of novel molecule-semiconductor devices.
Colloidal semiconductor nanocrystals become increasingly important in materials science and technology, due to their optoelectronic properties that are tunable by size. The measurement and understanding of their energy levels is key to scientific and technological progress. Here we review how the confined electronic orbitals and related energy levels of individual semiconductor quantum dots have been measured by means of scanning tunneling microscopy and spectroscopy. These techniques were originally developed for flat conducting surfaces, but they have been adapted to investigate the atomic and electronic structure of semiconductor quantum dots. We compare the results obtained on colloidal quantum dots with those on comparable solid-state ones. We also compare the results obtained with scanning tunneling spectroscopy with those of optical spectroscopy. The first three sections provide an introduction to colloidal quantum dots, and a theoretical basis to be able to understand tunneling spectroscopy on dots attached to a conducting surface. In sections 4 and 5 , we review the work performed on lead-chalcogenide nanocrystals and on colloidal quantum dots and rods of II-VI compounds, respectively. In section 6 , we deal with colloidal III-V nanocrystals and compare the results with their self-assembled counter parts. In section 7 , we review the work on other types of semiconductor quantum dots, especially on Si and Ge nanocrystals.
Charge transport in polymer- and oligomer-based semiconductor materials depends strongly on the structural ordering of the constituent molecules. Variations in molecular conformations influence the electronic structures of polymers and oligomers, and thus impact their charge-transport properties. In this study, we used Scanning Tunneling Microscopy and Spectroscopy (STM/STS) to investigate the electronic structures of different alkyl-substituted oligothiophenes displaying varied torsional conformations on the Au(111) surface. STM imaging showed that on Au(111) oligothiophenes self-assemble into chain-like structures, binding to each other via interdigitated alkyl ligands. The molecules adopted distinct planar conformations with alkyl ligands forming cis- or trans- mutual orientations. For each molecule, by using STS mapping, we identify a progression of particle-in-a-box-like states corresponding to the LUMO, LUMO+1 and LUMO+2 orbitals. Analysis of STS data revealed very similar unoccupied molecular orbital energies for different possible molecular conformations. By using density functional theory calculations, we show that the lack of variation in molecular orbital energies among the different oligothiophene conformers implies that the effect of the Au-oligothiophene interaction on molecular orbital energies is nearly identical for all studied torsional conformations. Our results suggest that cis-trans torsional disorder may not be a significant source of electronic disorder and charge carrier trapping in organic semiconductor devices based on oligothiophenes.
Trajectory-based mixed quantum-classical approaches to coupled
electron-nuclear dynamics suffer from well-studied problems such as the lack of
(or incorrect account for) decoherence in the trajectory surface hopping method
and the inability of reproducing the spatial splitting of a nuclear wave packet
in Ehrenfest-like dynamics. In the context of electronic non-adiabatic
processes, these problems can result in wrong predictions for quantum
populations and in unphysical outcomes for the nuclear dynamics. In this paper
we propose a solution to these issues by approximating the coupled electronic
and nuclear equations within the framework of the exact factorization of the
electron-nuclear wave function. We present a simple quantum-classical scheme
based on coupled classical trajectories, and test it against the full quantum
mechanical solution from wave packet dynamics for some model situations which
represent particularly challenging problems for the above-mentioned traditional
methods.
Electronic energy transfer (EET) from a donor to an acceptor is an important mechanism that controls the light har-vesting efficiency in a wide variety of systems, including artificial and natural photosynthesis and contemporary pho-tovoltaic technologies. The detailed mechanism of EET at short distances or large angles between the donor and ac-ceptor is poorly understood. Here the influence of the orientation between the donor and acceptor on EET is explored using a molecule with two nearly perpendicular chromophores. Very fast EET with a time constant of 120 fs is observed, which is at least forty times faster than the time predicted by Coulombic coupling calculations. Depolarization of the emission signal indicates that the transition dipole rotates through ca. 640, indicating the near orthogonal nature of the EET event. The rate of EET is found to be similar to structural relaxation rates in the photo-excited oligothiophene donor alone, which suggests that this initial relaxation brings the dyad to a conical intersection where the excitation jumps to the acceptor.
The theory of vibronic transitions in rare earth compounds is re-examined in the light of a more reliable representation for the ligand field Hamiltonian than the crude electrostatic model. General expressions that take into account the relevant contributions from the forced electric dipole and dynamic coupling mechanisms are derived for the vibronic intensity parameters. These include additional terms, from charge and polarizability gradients, which have not been considered in previous work. Emphasis is given to the relative signs of these various contributions. Under certain approximations these expressions may be conveniently written in terms of accessible ligand field parameters. A comparison with experimental values for the compounds Cs2NaEuCl6 and LiEuF4 is made and satisfactory agreement between theory and experiment is found. A discussion is given on the sensitivity of the calculated intensities to the values of radial integrals, interconfigurational energy differences and ligand field parameters that may be used. Finally, the problem in which a vibronic and an electronic level are in resonance, or near resonance, is analyzed. Suitable expressions to describe the effects of the even-rank components of the vibronic Hamiltonian are obtained. It is found that, depending on the strength of the vibronic interaction and the resonance conditions, the admixture between these two levels may lead to intensities of nearly equal values.
Transport of electrons in a single molecule junction is the simplest problem in the general subject area of molecular electronics. In the past few years, this area has been extended to probe beyond the simple tunnelling associated with large energy gaps between electrode Fermi level and molecular levels, to deal with smaller gaps, with near-resonance tunnelling and, particularly, with effects due to interaction of electronic and vibrational degrees of freedom. This overview is devoted to the theoretical and computational approaches that have been taken to understanding transport in molecular junctions when these vibronic interactions are involved. After a short experimental overview, and discussion of different test beds and measurements, we define a particular microscopic model Hamiltonian. That overall Hamiltonian can be used to discuss all of the phenomena dealt with subsequently. These include transition from coherent to incoherent transport as electron/vibration interaction increases in strength, inelastic electron tunnelling spectroscopy and its interpretation and measurement, affects of interelectronic repulsion treated at the Hubbard level, noise in molecular transport junctions, non-linear conductance phenomena, heating and heat conduction in molecular transport junctions and current-induced chemical reactions. In each of these areas, we use the same simple model Hamiltonian to analyse energetics and dynamics. While this overview does not attempt survey the literature exhaustively, it does provide appropriate references to the current literature (both experimental and theoretical). We also attempt to point out directions in which further research is required to answer cardinal questions concerning the behaviour and understanding of vibrational effects in molecular transport junctions.
We present density-functional and time-dependent density-functional studies of the ground, ionic, and excited states of a series of oligomers of thiophene. We show that, for the physical properties, the most relevant highest occupied and lowest unoccupied molecular orbitals develop gradually from monomer molecular orbitals into occupied and unoccupied broad bands in the large length limit. We show that band gap and ionization potentials decrease with size, as found experimentally and from empirical calculations. This gives credence to a simple tight-binding model Hamiltonian approach to these systems. We demonstrate that the length dependence of the experimental excitation spectra for both singlet and triplet excitations can be very well explained with an extended Hubbard-like Hamiltonian, with a monomer on-site Coulomb and exchange interaction and a nearest-neighbor Coulomb interaction. We also study the ground and excited-state electronic structures as functions of the torsion angle between the units in a dimer, and find almost equal stabilities for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsion angle turn out to be very low in energy, and therefore of great importance in describing even the room-temperature properties. At a torsion angle of 90° the hopping integral is switched off for the highest occupied molecular orbital levels because of symmetry, allowing a first-principles estimate of the on-site interaction minus the next-neighbor Coulomb interaction as it enters in a Hubbard-like model Hamiltonian.
The ultimate aim of molecular electronics is to understand and master single-molecule devices. Based on the latest results on electron transport in single molecules in solid-state devices, we focus here on new insights into the influence of metal electrodes on the energy spectrum of the molecule, and on how the electron transport properties of the molecule depend on the strength of the electronic coupling between it and the electrodes. A variety of phenomena are observed depending on whether this coupling is weak, intermediate or strong.
The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through
individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular
wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular
species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire
by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length
up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations
as one molecular unit after another is detached from the surface during stretching.
Understanding the influence of vibrational motion of the atoms on electronic transitions in molecules constitutes a cornerstone of quantum physics, as epitomized by the Franck-Condon principle of spectroscopy. Recent advances in building molecular-electronics devices and nanoelectromechanical systems open a new arena for studying the interaction between mechanical and electronic degrees of freedom in transport at the single-molecule level. The tunneling of electrons through molecules or suspended quantum dots has been shown to excite vibrational modes, or vibrons. Beyond this effect, theory predicts that strong electron-vibron coupling dramatically suppresses the current flow at low biases, a collective behaviour known as Franck-Condon blockade. Here we show measurements on quantum dots formed in suspended single-wall carbon nanotubes revealing a remarkably large electron-vibron coupling and, due to the high quality and unprecedented tunability of our samples, admit a quantitative analysis of vibron-mediated electronic transport in the regime of strong electron-vibron coupling. This allows us to unambiguously demonstrate the Franck-Condon blockade in a suspended nanostructure. The large observed electron-vibron coupling could ultimately be a key ingredient for the detection of quantized mechanical motion. It also emphasizes the unique potential for nanoelectromechanical device applications based on suspended graphene sheets and carbon nanotubes.
Self-assembly of molecules on surfaces is a route toward not only creating structures, but also engineering chemical reactivity
afforded by the intermolecular interactions. Dimethyldisulfide (CH3SSCH3) molecules self-assemble into linear chains on single-crystal gold surfaces. Injecting low-energy electrons into individual
molecules in the self-assembled structures with the tip of a scanning tunneling microscope led to a propagating chemical reaction
along the molecular chain as sulfur–sulfur bonds were broken and then reformed to produce new CH3SSCH3 molecules. Theoretical and experimental evidence supports a mechanism involving electron attachment followed by dissociation
of a CH3SSCH3 molecule and initiation of a chain reaction by one or both of the resulting CH3S intermediates.
The ability of a scanning tunneling microscope to manipulate single atoms is used to build well-defined gold chains on NiAl(110). The electronic properties of the one-dimensional chains are dominated by an unoccupied electron band, gradually developing from a single atomic orbital present in a gold atom. Spatially resolved conductance measurements along a 20-atom chain provide the dispersion relation, effective mass, and density of states of the free electron-like band. These experiments demonstrate a strategy for probing the interrelation between geometric structure, elemental composition, and electronic properties in metallic nanostructures.
The interplay between discrete vibrational and electronic degrees of freedom directly influences the chemical and physical properties of molecular systems. This coupling is typically studied through optical methods such as fluorescence, absorption and Raman spectroscopy. Molecular electronic devices provide new opportunities for exploring vibration-electronic interactions at the single molecule level. For example, electrons injected from a scanning tunnelling microscope tip into a metal can excite vibrational excitations of a molecule situated in the gap between tip and metal. Here we show how current directly injected into a freely suspended individual single-wall carbon nanotube can be used to excite, detect and control a specific vibrational mode of the molecule. Electrons tunnelling inelastically into the nanotube cause a non-equilibrium occupation of the radial breathing mode, leading to both stimulated emission and absorption of phonons by successive electron tunnelling events. We exploit this effect to measure a phonon lifetime of the order of 10 ns, corresponding to a quality factor of well over 10,000 for this nanomechanical oscillator.
Ultrathin insulating NaCl films have been employed to decouple individual pentacene molecules electronically from the metallic substrate. This allows the inherent electronic structure of the free molecule to be preserved and studied by means of low-temperature scanning-tunneling microscopy. Thereby direct images of the unperturbed molecular orbitals of the individual pentacene molecules are obtained. Elastic scattering quantum chemistry calculations substantiate the experimental findings.
We used a scanning tunneling microscope to probe the interactions between spins in individual atomic-scale magnetic structures.
Linear chains of 1 to 10 manganese atoms were assembled one atom at a time on a thin insulating layer, and the spin excitation
spectra of these structures were measured with inelastic electron tunneling spectroscopy. We observed excitations of the coupled
atomic spins that can change both the total spin and its orientation. Comparison with a model spin-interaction Hamiltonian
yielded the collective spin configuration and the strength of the coupling between the atomic spins.
The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission
spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by
difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented
sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction
parallel to the principal molecular axis, a quasi–one-dimensional band structure of a system of well-defined finite size develops
out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity
of the molecular crystal, and continuous bands with a large dispersion were observed.
The existence of one-dimensional (1D) electronic states in Cu/Cu(111) chains assembled by atomic manipulation is revealed by low-temperature scanning tunneling spectroscopy and density functional theory (DFT) calculations. Our experimental analysis of the chain-localized electron dynamics shows that the dispersion is fully described within a 1D tight-binding approach. DFT calculations confirm the confinement of unoccupied states to the chain in the relevant energy range, along with a significant extension of these states into the vacuum region.
Much current experimental research on transport in molecular junctions focuses on finite voltages, where substantial polarization-induced
nonlinearities may result in technologically relevant device-type responses. Because molecules have strong polarization responses
to changing charge state or external field, molecules isolated between electrodes can show strongly nonlinear current-voltage
responses. For small applied voltages (up to ∼0.3 volt), weak interaction between transporting electrons and molecular vibrations
provides the basis for inelastic electron tunneling spectroscopy. At higher voltages and for certain time scale regimes, strong
coupling effects occur, including Coulomb blockade, negative differential resistance, dynamical switching and switching noise,
current hysteresis, heating, and chemical reactions. We discuss a general picture for such phenomena that arise from charging,
strong correlation, and polarization (electronic and vibrational) effects in the molecule and at the interface.
A time-dependent approach is used to explore inelastic effects during electron transport through few-level systems. We study a tight-binding chain with one and two sites connected to vibrations. This simple but transparent model gives insight about inelastic effects, their meaning and the approximations currently used to treat them. Our time-dependent approach allows us to trace back the time sequence of vibrational excitation and electronic interference, the ibrationally introduced time delay and the electronic phase shift. We explore a full range of parameters going from weak to strong electron-vibration coupling, from tunneling to contact, from one-vibration description to the need of including all vibrations for a correct description of inelastic effects in transport. We explore the validity of single-site resonant models as well as its extension to more sites via molecular orbitals and the conditions under which multi-orbital, multi-vibrational descriptions cannot be simplified. We explain the physical meaning of the spectral features in the second derivative of the electron current with respect to the bias voltage. This permits us to nuance the meaning of the energy value of dips and peaks. Finally, we show that finite-band effects lead to electron back-scattering off the molecular vibrations in the regime of high-conductance, although the drop in conductance at the vibrational threshold is rather due to the rapid variation of the vibronic density of states. Comment: 38 pages, 14 figures
Vibronic interaction effects constitute a new field of investigation in the physics
and chemistry of molecules and crystals that combines all the phenomena and laws
originating from the mixing of different electronic states by nuclear displacements.
This field is based on a new concept which goes beyond the separate descriptions
of electronic and nuclear motions in the adiabatic approximation. Publications on
this topic often appear under the title of the lahn-Thller effect, although the area
of application of the new approach is much wider: the term vibronic interaction
seems to be more appropriate to the field as a whole.
The present understanding of the subject was reached only recently, during the
last quarter of a century. As a result of intensive development of the theory and
experiment, it was shown that the nonadiabatic mixing of close-in-energy electronic
states under nuclear displacements and the back influence of the modified
electronic structure on the nuclear dynamics result in a series of new effects in the
properties of molecules and crystals. The applications of the theory of vibronic interactions
cover the full range of spectroscopy [including visible, ultraviolet, infrared,
Raman, EPR, NMR, nuclear quadrupole resonance (NQR), nuclear gamma
resonance (NOR), photoelectron and x-ray spectroscopy], polarizability and
magnetic susceptibility, scattering phenomena, ideal and impurity crystal physics
and chemistry (including structural as well as ferroelectric phase transitions),
stereochemistry and instability of molecular (including biological) systems,
mechanisms of chemical reactions and catalysis.
The most interesting achievements of the theory of vibronic interactions are
related to structural phase transitions in crystals (or, in a wider sense, to structural
phase transformations in condensed media), the physics of impurity centers in
crystals, and spectroscopy of polyatomic systems. In particular, structural aspects
of the high-temperature superconductivity discovered recently have, beyond doubt,
a vibronic nature.
This volume emerged from investigations on the theory of vibronic interactions
carried out in collaboration with co-workers and colleagues at the Moldavian
Academy of Sciences over a period of more than 25 years. It is the first book on
this topic to be published for more than 15 years, during which time new ideas have
been developed and some problems of primary importance have been solved. The
authors hope that all main results on the theory of vibronic interactions in
molecules and crystals obtained until now are well represented in this book, with
the exception of some chemical and biological applications and some other special
problems. Compared with the Russian book on which this edition is based, this volume is more complete, in particular due to the addition of a survey of Green's
function approaches, the solutions of multimode and multicenter problems,
evaluations of additional interesting cases of optical and photoelectron spectra,
polarizabilities and birefringence, Peierls transitions, and a general model for
structural phase transitions in condensed media.
This book is mainly addressed to physicists - theoreticians and experimentalists
(researchers, professors, graduate students, etc.) - specialists in the field of
the structure and properties of molecules and crystals. However, this book may
also be useful for specialists in related fields, in particular for chemists, biologists,
and materials scientists; many new properties of materials have proved to be of
vibronic origin.
Kishinev, January 1989
L B. Bersuker
V. Z. Polinger
A general mathematical treatment of vibronic coupling of two electronic states in molecules and dimers is presented. The 2×2 matrix Hamiltonian which is derived is shown to reduce to two one-dimensional Hamiltonians provided a certain minimum amount of symmetry is present. Some general selection rules governing electronic transitions to these states from the ground state are obtained, showing that the spectral distribution in hot bands may differ considerably from that in normal bands when vibronic coupling occurs. A special model which considers the coupling to arise from a single mode of vibration is presented. Two limiting cases which can be treated approximately by perturbation theory are considered in detail. These are shown to give rise to a ``pseudo Jahn-Teller effect'' and to vibrational borrowing in the two different limits.
We consider the optical absorption of a model system consisting of a two-level ’’ion’’ interacting with a continuum of lattice vibrations. The two-level system represents localized electronic energy levels such as are found on transition metal ions. The separation of the electronic energies is taken to lie within the phonon band and the interaction with the lattice vibrations is linear. For a single band of phonons the absorption is shown to consist of three peaks—a homogeneously broadened central peak flanked by two sharp lines. The existence of three absorption peaks is in contrast to the predictions of either a static crystal field or of the two-level system interacting with a single harmonic oscillator. Our results reduce to the latter case when the phonon bandwidth becomes very narrow and to the former when the phonon bandwidth becomes very large.
We present a quantum-chemical study of the electronic spectra of the oligomers of thiophene. Geometries and vibrational force fields of the ground and excited electronic states are obtained by an updated version of the semiempirical quantum consistent force field/π electron method implemented to describe sulphur atoms and by abinitio Hartree–Fock and configuration interaction singles methods. The displacement parameters of totally symmetric modes are then obtained and used to model the vibrational structure of the electronic spectra. The contribution of sulphur atoms to the description of the excited state is predicted to be negligible both by abinitio and semiempirical methods which, conversely, indicate a close similarity of thiophene oligomers and polyenes. Based on the results of the simulated spectra a reassignment of some of the bands is proposed. It is shown that mode mixing upon excitation, and not large frequency changes, are responsible for the different Franck–Condon structure of the absorption and emission spectra. In addition, a vibronic coupling mechanism analogous to that active in simple polyenes is identified. It accounts for the ‘‘apparent’’ frequency increase of the most active ag mode upon excitation to the 1B state.
Self-localized nonlinear excitations (solitons, polarons, and bipolarons) are fundamental and inherent features of quasi-one-dimensional conducting polymers. Their signatures are evident in many aspects of the physical and chemical properties of this growing class of novel materials. As a result, these polymers represent an opportunity for exploring the novel phenomena associated with topological solitons and their linear confinement which results from weakly lifting the ground-state degeneracy. The authors review the theoretical models that have been developed to describe the physics of polyacetylene and related conducting polymers and summarize the relevant experimental results obtained for these materials. An attempt is made to assess the validity of the soliton model of polyacetylene and its generalization to related systems in which the ground-state degeneracy has been lifted.
The exact resonant-tunneling transmission probability for an electron interacting with phonons is presented in the limit that the elastic coupling to the leads is independent of energy. The phonons produce transmission sidebands but do not affect the integrated transmission probability or the escape rate of the electron from the resonant site. In the Appendixes, we evaluate the Green function that appears in the expression for the transmission probability.
Linear Au chains two to 20 atoms long were constructed on a NiAl(110) surface via the manipulation of single atoms with a scanning tunneling microscope. Differential conductance (dI/dV) images of these chains reveal one-dimensional electronic density oscillations at energies 1.0 to 2.5 eV above the Fermi energy. The origin of this delocalized electronic structure is traced to the existence of an electronic resonance measured on single, isolated Au atoms. Variations in the wavelength in dI/dV images of an eleven-atom chain taken at different energies revealed an effective electronic mass of 0.4+/-0.1 times the mass of a free-electron.
Molecular conductance junctions are structures in which single molecules or small groups of molecules conduct electrical current
between two electrodes. In such junctions, the connection between the molecule and the electrodes greatly affects the current-voltage
characteristics. Despite several experimental and theoretical advances, including the understanding of simple systems, there
is still limited correspondence between experimental and theoretical studies of these systems.
A scanning tunneling microscope was used to study the electron transport through individual copper phthalocyanine molecules adsorbed on an ultrathin Al(2)O(3) film grown on a NiAl(110) surface. The differential conductance spectra display series of equally spaced features, which are attributed to vibronic states of individual molecules. The coupling of the electron current to the vibronic modes was observed to depend on the structures of the adsorbed molecules. Vibronic features were not observed for molecules adsorbed on the bare NiAl(110) surface due to spectral broadening.
End states--the zero-dimensional analogs of the two-dimensional states that occur at a crystal surface--were observed at the ends of one-dimensional atom chains that were self-assembled by depositing gold on the vicinal Si(553) surface. Scanning tunneling spectroscopy measurements of the differential conductance along the chains revealed quantized states in isolated segments with differentiated states forming over end atoms. A comparison to a tight-binding model demonstrated how the formation of electronic end states transforms the density of states and the energy levels within the chains.
Broad Gaussian line shapes are observed in scanning tunneling spectroscopy of single, localized electronic states induced by Cl vacancies in ultrathin NaCl films on Cu surfaces. Using a simple inelastic resonance tunneling model, we show that the observed broad line shapes are caused by a strong coupling between the localized state and the optical phonons in the film. The parameters for the model are obtained from density functional calculations, in which the occupation of the vacancy state temporarily taking place in the experiment has also been accounted for.
Scanning tunneling spectroscopy on single naphthalocyanine molecules adsorbed on an ultrathin aluminum oxide film exhibits electron-vibronic coupling that varies with the position of tunneling over the molecule. The spectra at different positions are composed of several series of equally spaced peaks, which are interpreted as progression of progressions of molecular vibrational modes. The spatial variations correlate with the molecular orbital structure, revealing spatially dependent electron-vibronic coupling and selective vibrational excitation.
The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that was manipulated and probed by low-temperature scanning tunneling microscopy. When adsorbed on an ultrathin insulating film, the molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunneling current. The tautomerization reaction can be probed by resonant tunneling through the molecule and is expressed as considerable changes in the conductivity of the molecule. We also demonstrated a coupling of the switching process so that the charge injection in one molecule induced tautomerization in an adjacent molecule.
The charge transport mechanism of a wire can be revealed by how its electrical resistance varies with length. We have measured
the resistance and current-voltage characteristics of conjugated molecular wires ranging in length from 1 to 7 nanometers,
connected between metal electrodes. We observe the theoretically predicted change in direct-current transport from tunneling
to hopping as a function of systematically controlled wire length. We also demonstrate that site-specific disruption of conjugation
in the wires greatly increases resistance in the hopping regime but has only a small effect in the tunneling regime. These
nanoscale transport measurements elucidate the role of molecular length and bond architecture on molecular conductivity and
open opportunities for greater understanding of electrical transport in conjugated polymer films.
We calculate the electronic transport through a molecular dimer, in which an excess electron is delocalized over equivalent monomers, which can be locally distorted. In this system the Born-Oppenheimer approximation breaks down resulting in quantum entanglement of the mechanical and electronic motion. We show that pseudo Jahn-Teller (pJT) dynamics of the molecule gives rise to conductance peaks that indicate this violation. Their magnitude, sign and position sharply depend on the electro-mechanical properties of the molecule, which can be varied in recently developed three-terminal junctions with mechanical control. The predicted effect depends crucially on the degree of intramolecular delocalization of the excess electron, a parameter which is also of fundamental importance in physical chemistry.
- I B Bersuker
- V Z Polinger
Bersuker, I. B. & Polinger, V. Z. Vibronic Interactions in Molecules and Crystals
(Springer, 1989).
Molecular transport junctions: Vibrational effects
- M Galperin
- M A Ratner
- A Nitzan
Galperin, M., Ratner, M. A. & Nitzan, A. Molecular transport junctions:
Vibrational effects. J. Phys. Condens. Matter 19, 103201 (2007).