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Controlling on-surface molecular diffusion behaviors by functionalizing the
organic molecules with tert-butyl groups
Qiang Sun, Chi Zhang, Zhiwen Li, Kai Sheng, Huihui Kong et al.
Citation: Appl. Phys. Lett. 103, 013103 (2013); doi: 10.1063/1.4811353
View online: http://dx.doi.org/10.1063/1.4811353
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Controlling on-surface molecular diffusion behaviors by functionalizing
the organic molecules with tert-butyl groups
Qiang Sun,
1
Chi Zhang,
1
Zhiwen Li,
2
Kai Sheng,
1
Huihui Kong,
1
Likun Wang,
1
Yunxiang Pan,
1
Qinggang Tan,
1
Aiguo Hu,
2
and Wei Xu
1,a)
1
College of Materials Science and Engineering, Key Laboratory for Advanced Civil Engineering Materials
(Ministry of Education), Tongji University, Caoan Road 4800, Shanghai 201804, People’s Republic of China
2
School of Materials Science and Engineering, East China University of Science and Technology,
Meilong Road 130, Shanghai 200237, People’s Republic of China
(Received 5 April 2013; accepted 19 May 2013; published online 1 July 2013)
We have performed the systematic studies on three structurally similar aromatic molecules with
different functional groups on a Cu(110) surface and investigated their on-surface molecular
diffusion behaviors by the interplay of scanning tunneling microscopy imaging and density
functional theory calculations. We have found that the tert-butyl groups could significantly affect
the molecular adsorption geometries and moreover the mobility of the molecules on the surface.
These findings could give further insights into the understanding of diffusion behaviors of organic
molecules specifically with tert-butyl groups on surfaces. V
C2013 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4811353]
The dynamic behaviors of organic molecules on solid
surfaces always play a vital role in surface physicochemical
processes. Surface diffusion, as one of the preliminary
dynamic behaviors of organic molecules for achieving more
complex phenomena including film growth, heterogeneous
catalysis, on-surface chemical reaction and the realization of
nanodevices with advanced functions, has received a lot of
attentions.
1–7
In general, by delicately controlling the molec-
ular structures and choosing the appropriate substrates, dif-
ferent molecular diffusion behaviors could be regulated on
surfaces. For example, the isotropic mobility could be turned
into unidirectional motions by modifying the adsorbates with
fullerene “wheels”
6
or two sequentially moving thiol sub-
strate linkers;
4
later on, the fullerene molecules were shown
to diffuse along the ½1
10direction of Pd(110) surface result-
ing from the confinement of the substrate lattice.
7
Moreover,
the tert-butyl groups were reported to be capable of regulat-
ing the diffusion behaviors of organic molecules by increas-
ing the distance between surface and p-system of the
molecules,
8
or controlling the registry between the molecules
and the substrate.
1
Although previous studies have demon-
strated some advancements in these aspects, it is, however,
still of general interest to systematically study the influence
of functional groups on the diffusion behaviors of organic
molecules on surfaces, and to get further insights and deeper
understanding on various dynamic behaviors occurring in a
more complicated situation.
In this work, we focus on the influence of tert-butyl
groups on the mobility of organic molecules on a Cu(110)
surface.
1,8
We have designed three aromatic molecules,
(Z)-1,6-di(naphthalen-2-yl)hexa-3-en-1,5-diyne, tetraki-
s(phenylethynyl)ethane and (Z)-1,6-bis-(4-(tert-butyl)phe-
nyl)hexa-3-en-1,5-diyne, shortened as DNHD, TPEE, and
BtPHD, respectively (the structure models are shown in
Fig. 1). The TPEE molecule (C
34
H
20
) and the BtPHD mole-
cule (C
26
H
28
) are both derived from the DNHD molecules
(C
26
H
16
). The TPEE has similar functional groups as DNHD
but with larger molecular weight, and the BtPHD has similar
molecular weight as DNHD but with different functional
group (the tert-butyl-phenyl groups replacing the naphthyl
groups). From an interplay of high-resolution ultra-high vac-
uum (UHV) scanning tunneling microscopy (STM) imaging
and density functional theory (DFT) calculations, we have
shown that both the DNHD and the TPEE molecules adopted
planar adsorption geometries and exhibited high mobility on
the Cu(110) surface, while the BtPHD molecule with termi-
nal tert-butyl groups preferred a tilted geometry and, surpris-
ingly, demonstrated distinctly low mobility comparing to
that of the DNHD and TPEE molecules. These findings pro-
vide further insights into a complementary understanding on
how the tert-butyl groups could influence the molecular
adsorption behaviors and moreover the mobility of mole-
cules on the surface, and also indicate that the tert-butyl
groups could be good candidates for delicately controlling
the dynamic behaviors of aromatic molecules on surfaces.
The STM experiments were performed in an UHV
chamber (base pressure 1 10
10
mbar) equipped with a
variable-temperature “Aarhus-type” STM purchased from
SPECS.
9,10
The compounds were loaded into three separated
glass crucibles in a molecular evaporator. After the system
was thoroughly degassed, the compounds were deposited by
thermal sublimation onto a Cu(110) substrate. The sample
was thereafter transferred within the UHV chamber to the
STM, where measurements were carried out at 100 K. All
the calculations were carried out in the framework of DFT
by using the Vienna ab initio simulation package (VASP).
11,12
The projector augmented wave method was used to describe
the interaction between ions and electrons.
13,14
The atomic
structures were relaxed using the conjugate gradient algo-
rithm scheme as implemented in the VASP code until the
forces on all unconstrained atoms were 0.03 eV/A
˚.
a)
Author to whom correspondence should be addressed. Electronic mail:
xuwei@tongji.edu.cn
0003-6951/2013/103(1)/013103/4/$30.00 V
C2013 AIP Publishing LLC103, 013103-1
APPLIED PHYSICS LETTERS 103, 013103 (2013)
As shown in Fig. 1(a), after deposition of the DNHD
molecules on Cu(110) at low temperature (170 K) we
found that the isolated molecules were distributed on the sur-
face and appeared as heart shapes with two elliptical lobes
and one round protrusion (cf. the close-up STM image),
which suggested that the DNHD molecule adopted a flat-
lying geometry on Cu(110). We assign the two elliptical
lobes to the naphthyl groups and the round protrusion to the
vinyl group as compared with the gas-phase relaxed molecu-
lar model. The DNHD molecules mainly adsorbed with the
symmetry axis aligning along the ½1
10direction of the sub-
strate. Interestingly, during scanning we found that there
were some blurred heart shapes in the large-scale STM
images, and these blurred shapes were also mainly along the
½1
10direction of the substrate and their widths were equal
to that of the DNHD molecule as indicated by the parallel
green lines in Fig. 1(a). It should be noted that the seemingly
stationary molecules in Fig. 1(a) were also found to be mo-
bile as reflected in the sequential STM images by keeping
scanning on the same region (not shown). We could thus
infer that the blurs are attributed to the trajectories of the mo-
bile DNHD molecules diffusing along the ½1
10direction
resulting from the much higher mobility of the molecules on
the surface comparing with the scanning speed (the typical
time for recording one frame is about 10 s), which is a com-
mon phenomenon in STM experiments. Note that the molec-
ular diffusion direction is unidirectional and different from
the scanning direction (from left to right); thus, the tip-
induced diffusion behavior could be excluded.
To investigate the influence of molecular weight on the
molecular diffusion behaviors, we deposited the TPEE mole-
cules (which could be roughly regarded as functionalizing the
DNHD molecules with two more aromatic legs) on Cu(110)
at 170 K and performed the STM experiments in the same
conditions as for NDHD molecules. As shown in Fig. 1(b),
similarly, the isolated TPEE molecules were distributed on
the surface at a relatively low coverage and appeared as four
peripheral lobes and one central protrusion (cf. the close-up
STM image), and we assigned the four lobes to the phenyl
groups and the center protrusion to the vinyl group as com-
pared with the gas-phase relaxed molecular model. Just like
the case of the DNHD molecules, the TPEE molecules pre-
ferred to lie flat on the Cu(110) surface with the long symme-
try axis aligning along the ½1
10direction of the substrate.
Moreover, though two more aromatic “legs” have been added
onto the TPEE molecule, there were still blurred trajectories
moving along the ½1
10direction with the same width as the
TPEE molecules as indicated by the parallel green lines in
Fig. 1(b), which was the sign that the TPEE molecules were
still highly mobile on the Cu(110) surface.
To further explore the influence of functional groups on
the molecular diffusion behaviors, we deposited the BtPHD
molecules (which replaced the naphthyl groups of the DNHD
molecule by the tert-butyl-phenyl groups) on Cu(110) at
170 K and performed the STM experiments under the same
conditions. As shown in Fig. 1(c), the isolated BtPHD mole-
cules exhibited in STM images as two bright protrusions cor-
responding to high electron tunneling probability through the
tert-butyl groups to the substrates.
15–17
Since the tert-butyl
groups dominated the STM image of a single BtPHD mole-
cule, the vinyl group of the molecule just appeared as dark
protrusion (cf. the close-up STM image and the gas-phase
relaxed molecular model). The BtPHD molecules also pre-
ferred to isolatedly distribute on Cu(110) at a relative low
coverage, yet, mainly adsorbed with the symmetry axis align-
ing perpendicular to the ½1
10direction. More interestingly,
unlike the cases of the DNHD and TPEE molecules, there
were no blurred trajectories observed during the scanning
process, which was a sign of relatively low mobility of
BtPHD molecules. From the experiment, it was obvious that
the tert-butyl groups could significantly affect the adsorption
FIG. 1. STM images recorded with the same scanning parameter after depositing (a) DNHD, (b) TPEE, and (c) BtPHD on Cu(110). The close-up STM images
and the gas-phase relaxed molecular models (H: white, C: gray) were shown in the upper-right and lower-right panels, respectively. Scanning condition:
I
t
¼0.7 nA; V
t
¼2500 mV; T ¼100 K.
FIG. 2. The close-up STM images and DFT optimized adsorption geome-
tries of (a) DNHD, (b) TPEE, and (c) BtPHD molecules on Cu(110). The
middle panels are top-view models and the lower panels are side-view
models.
013103-2 Sun et al. Appl. Phys. Lett. 103, 013103 (2013)
geometry and the mobility of the BtPHD molecule on
Cu(110).
DFT calculations have been performed to understand
the adsorption geometries of three molecules on Cu(110) at
atomic scale. The optimized models of DNHD, TPEE, and
BtPHD molecules on the Cu(110) surface were depicted in
Fig. 2. As seen from the models both the DNHD and the
TPEE molecules prefer to adopt flat-lying configurations,
which is well consistent with the STM results. While, the
BtPHD molecule tends to adsorb with a non-planar configu-
ration in which the carbon atoms adjacent to the phenyl
groups are in close proximity to the substrate, while the vinyl
and tert-butyl-phenyl groups are tilted upwards as clearly
illustrated in the side view of the model (cf. Fig. 2(c)). The
STM and DFT results are in good agreement and match well
with the previous studies that organic molecules mainly con-
stituted by the aromatic p-conjugated groups tend to adopt
planar geometries on surfaces,
18–20
and the tert-butyl groups
could influence the molecular adsorption geometries by lift-
ing up the molecules from surfaces.
8
To get further insights into the different diffusion behav-
iors of these three molecules on Cu(110), we have also calcu-
lated the potential energy profiles for the diffusions of three
molecules along the ½1
10direction as illustrated in Fig. 3.In
the calculations, several stable configurations along the ½1
10
direction between two adjacent most stable adsorption sites
were calculated, and interpreted as intermediate states for the
molecular diffusions.
15,21
The DNHD and TPEE molecules
have very close diffusion barriers determined to be 0.195 eV
and 0.175 eV, respectively, while the BtPHD molecule has a
larger barrier of 0.325 eV. We can also estimate the rate con-
stant of three molecules in the experimental condition
(100 K) based on the Arrhenius equation
4,22
k¼AeEa=RT:(1)
By supposing a typical prefactor A of 10
13
s
1
, we obtain an
estimated rate constant of 1.49 10
3
s
1
for DNHD,
1.51 10
4
s
1
for TPEE, and 4.17 10
4
s
1
for BtPHD.
The difference of nearly 8 orders of magnitude between the
mobile and the immobile molecules indicates that at 100 K
the DNHD and TPEE molecules could be highly mobile
while BtPHD remains motionless, which is in excellent
agreement with the STM findings.
From the STM experiments and DFT calculations, it is
clearly seen that the mobility of the DNHD molecule could
FIG. 3. The potential energy profiles for the diffusion of (a) DNHD, (b) TPEE, and (c) BtPHD molecules on Cu(110) as a function of the displacement along
the ½1
10direction. The two zero points of the relative energy curves (at d ¼0A
˚and d ¼2.56 A
˚) correspond to the two adjacent most stable sites separated by
the lattice constant of Cu(110) along the ½1
10direction. The maxima of the relative energy curves (at d ¼1.42 A
˚) correspond to the transition states.
013103-3 Sun et al. Appl. Phys. Lett. 103, 013103 (2013)
be significantly changed by functionalizing with the tert-
butyl groups. Two possible points can be singled out with
respect to the decreased mobility of the BtPHD molecule on
Cu(110): (1) the tert-butyl groups are in a specific registry
with Cu(110) to lock the molecule on the surface
1
and (2)
the tilted molecular adsorption geometry caused by the tert-
butyl groups inhibits the surface diffusion behavior since the
flat-lying TPEE molecule (with larger molecular weight)
could freely diffuse on the surface. Note that the case for the
BtPHD is different from the one showing that the mobility of
aromatic molecules could be increased when lifting the
whole molecule away from the substrate by functionalizing
with tert-butyl groups.
8
In our case, only one side of the
BtPHD molecule was lifted resulting in a seesaw-like situa-
tion. This study has thus given us a complementary under-
standing on the influence of the tert-butyl groups on the
diffusion behaviors of aromatic molecules on surfaces.
In conclusion, by combining the high-resolution UHV-
STM imaging and DFT calculations, we have systematically
investigated the adsorption and diffusion behaviors of three
aromatic molecules on Cu(110) at the atomic scale. These
results reveal that the tert-butyl groups could greatly influ-
ence the adsorption geometries and mobility of aromatic
molecules on the surface, and thus could be one of the good
candidates as functional group to control the molecular
dynamic behaviors on surfaces. Further works could be
explored to generalize the application of the tert-butyl group
as a modulator on other molecular systems to get a deeper
understanding on the influence of the tert-butyl groups in
more sophisticated situations.
The authors acknowledge the financial supports from the
National Natural Science Foundation of China (21103128),
the Program for New Century Excellent Talents in
University (NCET-09-0607), the Shanghai Pujiang Program
(11PJ1409700), the Shanghai “Shu Guang” project supported
by Shanghai Municipal Education Commission and Shanghai
Education Development Foundation (11SG25), the
Fundamental Research Funds for the Central Universities, the
Research Fund for the Doctoral Program of Higher Education
of China (20120072110045).
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