View Article Online / Journal Homepage / Table of Contents for this issue

ChemComm

Dynamic Article Links

Cite this: Chem. Commun., 2011, 47, 11312–11314
www.rsc.org/chemcomm

COMMUNICATION

Published on 16 September 2011. Downloaded by Tsukuba Daigaku on 04/11/2014 01:28:32.

Scanning tunneling microscopy/spectroscopy on self-assembly of aglycine/Cu(111) nanocavity arraywz
Ken Kanazawa, Atsushi Taninaka, Hui Huang, Munenori Nishimura, Shoji Yoshida,
Osamu Takeuchi and Hidemi Shigekawa*
Received 31st March 2011, Accepted 5th September 2011DOI: 10.1039/c1cc11829c
The step-by-step analysis of a hierarchical self-assembly revealed
the incorporation of nanocavity blocks in a metastable orientation
to stabilize the organized array. The conﬁnement of 2D electrons
by a quantum corral was veriﬁed. Furthermore, manipulation of
an isolated C60 molecule was realized using nanocavities of B1.3 nm
diameter as a template.
The engineering of nanostructures through the self-assembly oforganic molecules is a key factor in the development of futurefunctional devices.1 Because of the advantages of real-spaceanalysis with atomic resolution, adsorbed molecules on solid
surfaces have been extensively studied by scanning tunnelingmicroscopy/spectroscopy (STM/STS), which provides valuable
information not only on structures and formation processes2,3
but also on their functional properties originating from theirmolecular and surface electronic structures.4 Furthermore, STM
enables the manipulation of molecular structures and functions,5
providing information on their dynamical processes of interest
from fundamental and practical viewpoints.
In the interactive dynamics of molecules on a solid surface,footprint chirality, i.e., the chirality determined after the adsorptionof molecules, plays an important role in providing a driving force
to realize functions by multistep processes via the formation ofmultimer units.3,6 Among the naturally occurring a-amino acids,
glycine is the only molecule that does not have chirality, althoughenantiomeric isomers appear on a metal at room temperature (RT)through the dissociation of hydrogen from the carboxyl group,
depending on the freedom of the directional relationship between
the two groups in the adsorbed form (molecules in R- andS-trimers in Fig. 1(b)).7 Since only two footprints with the sameformation energy are created, glycine provides a simple example tostudy the interactive dynamics in self-assembly.
Here, we present the results obtained by STM/STS for a
glycine/Cu(111) self-assembly.

Institute of Applied Physics, University of Tsukuba, 305-8573
Tsukuba, Japan. E-mail: hidemi@ims.tsukuba.ac.jp;
Web: http://dora.bk.tsukuba.ac.jp; Fax: +81 29 853 6469;
Tel: +81 29 853 6469
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc11829cz This article is part of the ChemComm ‘Molecule-based surface
chemistry’ web themed issue.

11312

Chem. Commun., 2011, 47, 11312–11314

Fig. 1 (a) STM image of isolated glycine trimers (sample voltage:Vs = +0.5 V, tunneling current: It = 1.0 nA). r and l indicate two
types of enantiomeric trimer. (b) Magniﬁed STM images of the trimers A
and B in (a) and their schematic illustrations ( : H, : O, : N, : C).
Solid and dotted green triangles are drawn for comparison. (c) STM
images of step-by-step homochiral-ring-formation of glycine trimers.

After molecular deposition on a clean Cu(111) surface, thesample was annealed at 350 K for 1 h.7 Then, STM/STS measurements were performed in ultrahigh vacuum (o1  108 Pa) at 5 K
using a tungsten tip.
Fig. 1(a) shows an STM image of a surface covered with athin layer of glycine molecules (o0.2 molecular layer (ML)).
Some triangular structures can be observed. Each side of thetriangle is about 1 nm, which is much larger than the size of asingle glycine molecule. The triangles consist of three brightspots with the same brightness surrounded by regions of depression that are darker than the original substrate. By considering
their directional relationships with the threefold rotational
symmetry of the Cu(111) surface, the triangular structures are
divided into two types with a mirror plane along the [112% ]direction labeled as r and l in Fig. 2(a). Our STM images
indicate the existence of enantiomeric isomers forming the
supramolecular trimers, i.e., each triangular structure consistsof three homochiral glycinate molecules, resulting in the twotypes described above. By considering the threefold rotational
symmetry of the substrate and the hydrogen bonding between
the amino and carboxyl groups, which have been observed in
the self-assembly of glycine/Cu(100),6,8 schematic structures of
the two types of homochiral glycinate trimers (R- and S-trimers)
were developed. The methylene groups and the Cu areas near theThis journal is c The Royal Society of Chemistry 2011

Published on 16 September 2011. Downloaded by Tsukuba Daigaku on 04/11/2014 01:28:32.

View Article Online

Fig. 3 (a) dI/dV–V and I–V (inset) obtained at the substrate (S) andthe center of a nanocavity (G) (set point: Vs = 0.5 V, It = 0.5 nA).
(b) and (c) dI/dV maps for the voltage range of 0.2 to +0.8 V (setpoint: Vs = 0.25 V, It = 0.5 nA, grid: 128  128).

Fig. 2 (a) STM image of self-assembled network of glycine trimers (Vs =0.5 V, It = 0.5 nA) and the cross-section along X–Y. (b) High-resolution
STM images of r and l in (a), and (c) schematic of the moleculararrangement of structure r ( : H, : O, : N, : C). The green dottedlines in the box show the hydrogen bonds between molecules.

carboxyl groups are attributed to the brightest points and theregions of depression, respectively, which is discussed in ESI.w
Fig. 1(c) shows the homochiral-ring formation of the glycine
trimers observed, suggesting the formation of a higher-order
structure based on the trimer units. In fact, for a larger coverageof glycine molecules (B0.5 ML), a network of trimers appearedthat formed an array of nanoscale cavities of B1.3 nm diameter(Fig. 2(a)). The structure has 2O13  2O13-R13.91 periodicity.
As shown by the domains r and l in Fig. 2(a) and theirmagniﬁcations (Fig. 2(b)), there are two structures with a [112% ]mirror plane, which consist of circularly ordered trimers withhomochiralities indicated by r and l in Fig. 1(b). The enantioselectivity observed in the formation of trimer units also plays animportant role in the growth of the large-scale network.
For the 2O13  2O13 structure previously reported, a doublering model was proposed.9 However, as shown by the crosssection in Fig. 2(a), both the inside and outside regions of thenanocavity have similar heights, suggesting that the bottom
surface in the nanocavity is also a bare Cu surface. Since STM
images depend on the bias voltage, we carried out STS to clarify
the molecular structure observed in Fig. 2. Fig. 3(a) shows dI/
dV–V curves obtained in the substrate Cu(111) area (S) and at thecenter of a nanocavity (G). The two curves are diﬀerent; however,the value of Vs E 0.4 V, from which the diﬀerence between thetwo curves originates, is close to the minimum energy of the freeelectron-like Cu(111) surface state,10 suggesting that both regions
are bare Cu surfaces and that the diﬀerence in Fig. 3(a) is due tothe conﬁnement of free electrons in the nanocavity.
Fig. 3(b) and (c) show dI/dV images obtained at various
sample bias voltages. As expected, the dI/dV maps clearly reﬂect
the electronic structures of the Cu(111) surface state conﬁned byThis journal is c The Royal Society of Chemistry 2011a single ring. At Vs = B+0.8 V, the center of the nanocavity isdark with a bright circular edge with a diameter smaller than thatof the nanocavity, providing an apparent image of the double-ringmodel; however, at Vs = B+0.1 V, a bright state appears in
the nanocavity. These are the second and ground states of theconﬁned electrons, respectively (see ESIw).
Fig. 2(c) shows the structural model for the network.
Neighboring trimers represented by blue and red circles
exhibit 1801 rotational symmetry. Since there are few isolated
1801-rotated r and l trimers, as shown in Fig. 1(a), the isolated
trimers are initially stabilized in accordance with the C3v
symmetry of the Cu(111) substrate surface. In the 1801
rotational arrangement, each glycinate has four hydrogenbonds between the amino and carboxyl groups, enabling intraand intertrimer coupling (dotted green lines in Fig. 2(c)) andproducing strong interactions.
In the self-assembly of (RS)-3-pyrroline-2-carboxylic acid on
Cu(110), the molecular chirality and adsorption footprints
appear to play a similar role in organization despite their diﬀerent
conformation energies.3 The results obtained for glycine/Cu(111)strongly indicate the importance of the footprint concept also
in the hierarchical self-assembly. Understanding the interactive dynamics at both microscopic and nonlocal scales isimportant, and the step-by-step analysis, for example shown in
Fig. 1(c), is eﬀective for obtaining such an understanding.
Finally, we demonstrate the ability of the network structure as atemplate for fullerene molecules. Fullerenes and their derivatives
on metal electrodes are prospective building blocks for functional
nanostructures. However, at a low coverage, they are usually somobile on the terraces of a metal surface that they easily diﬀuse
towards step edges and form condensed structures.11,12 Therefore,
it is diﬃcult to obtain the isolated molecular condition that isessential for achieving molecular devices.13 To overcome thisinherent problem, the use of templates has been studied.14 The
diameter of the nanocavities in the network is about 1.3 nm,
enabling the use of the nanocavity structure to realize a structurewith isolated C60 molecules on a Cu(111) surface.
Fig. 4(a) is an STM image of C60 molecules deposited on aglycine network structure at RT. Unlike the case without aglycine template, most of the C60 molecules were adsorbed on
the terraces. As clearly shown in the magniﬁed image in Fig. 4(b),each molecule is adsorbed near the center of the nanocavity,
Chem. Commun., 2011, 47, 11312–11314

11313

Published on 16 September 2011. Downloaded by Tsukuba Daigaku on 04/11/2014 01:28:32.

View Article Online

Fig. 4 (a) STM image of C60 molecules on a glycine/Cu(111) structure(Vs = 0.5 V, It = 0.5 nA). (b) Magniﬁed image of the square in (a).
(c) dI/dV–V curves obtained on isolated C60 (set point: +2.0 V, 1.0 nA).

suggesting that C60 molecules are in direct contact with the Cu
substrate surface. In fact, STS curves measured on the C60
molecules (Fig. 4(c)) exhibit a similar characteristic to thatobserved for C60 molecules adsorbed at a Cu(111) step edge.12,15
Fig. 5(a) shows the existence of two diﬀerent states for the C60
molecules adsorbed on the template. They are distinguished asbrighter protrusions (B1 and B2) and darker protrusions (D1 andD2) in Fig. 5(a), respectively. The diﬀerence in brightness
between the two states is clearer in the cross-section shown in
Fig. 5(c). The ratio of the number of molecules with the brightand dark states was 2 : 3, indicating that the darker state is themajor and more stable state. As shown in the dI/dV image inFig. 5(b), the molecule with the darker state has threefold
rotational symmetry on the Cu(111) substrate. Therefore, eachdarker C60 molecule is adsorbed with its six-membered ringfacing the substrate as schematically shown in Fig. 5(d), whichis considered to be more stable than the conﬁguration of theother state. The existence of two states due to a diﬀerence in themolecular orientation has also been reported for a molecularlayer of condensed C60 on a Cu(111) surface.12
We manipulated isolated C60 using STM (Fig. 5(e)). We
scanned an applied bias voltage with the STM tip placed above
C60 in the brighter state, indicated by an arrow in Fig. 5(e). Asshown in the I–V curve in Fig. 5(f), the tunneling current
suddenly dropped at Vs E +2.3 V, indicating a change in themolecular state. According to the cross-section along the dottedline in Fig. 5(e) obtained before and after the excitation, themolecule indicated by an arrow underwent a transformation to
the darker state, which is considered to be caused by the rotationof the molecule in the cavity (Fig. 5(g)). In this study, we onlysucceeded in inducing the transformation in one direction
because of the high stability of the darker state. If a molecule
such as an endohedral fullerene with an ellipsoid shape is used,
the molecule will be located at a noncentral position in thenanocavity and may be reversibly controlled between diﬀerent
states, making the structure more applicable for molecular devices.
In conclusion, the formation of a quantum dot array and thesuccessful manipulation of individual C60 show the potential ofthis system. The active use of the quantized states in thenanocavity with the combination of guest molecule may provide
additional functions to realize electronic/optical devices.

Notes and references

Fig. 5 (a) and (b) STM (Vs = +1.5 V, It = 0.5 nA) and dI/dV (Vs =1.5 V, It = 0.5 nA) images of the two states of adsorbed C60 molecules,indicated by B1 and B2 (bright) and by D1 and D2 (dark), respectively.
(c) Cross-section along the red dashed line in (a). (d) Schematic illustrationof the darker C60 molecule in the glycine nanocavity. (e) STM images
obtained before and after excitation. (f) I–V curve obtained for the molecule
indicated by an arrow in (e). (g) Cross-sections obtained along the dottedline in (e) before (blue) and after (red) excitation (I–V measurement).

11314

Chem. Commun., 2011, 47, 11312–11314

1 C. Joachim, J. Gimzewski and A. Aviram, Nature, 2000, 408, 541;
J. Barth, Annu. Rev. Phys. Chem., 2007, 58, 175.
2 J. Elemans, S. Lei and S. Feyter, Angew. Chem., Int. Ed., 2009,
48, 7298; O. Ivasenko, J. Macleod, K. Chernichenko,
E. Balenkova, R. Shapanchenko, V. Neuajdenko, F. Rosei andD. Perepichka, Chem. Commun., 2009, 1192.
3 L. Pérez-Garcı́a and D. Amabilino, Chem. Soc. Rev., 2007, 36, 941;
M. Forster, M. Dyer, S. Barrett, M. Persson and R. Raval, J. Phys.
Chem. C, 2011, 115, 1180.
4 R. Temirov, S. Soubatch, A. Luican and F. S. Tautz, Nature, 2006,444, 350; T. Komeda, H. Isshiki, J. Liu, Y. Zhang, N. Lorente, K. Katoh,B. Breedlove and M. Yamashita, Nat. Commun., 2011, 2, 217.
5 W. Ho, J. Chem. Phys., 2002, 117, 11033; Y. Sainoo, Y. Kim,T. Okawa, T. Komeda, H. Shigekawa and M. Kawai, Phys. Rev.
Lett., 2005, 95, 246102.
6 K. Kanazawa, A. Taninaka, O. Takeuchi and H. Shigekawa, Phys.
Rev. Lett., 2007, 99, 216102.
7 V. Efstathiou and D. Woodruﬀ, Surf. Sci., 2003, 531, 304.
8 K. Kanazawa, Y. Sainoo, Y. Konishi, S. Yoshida, A. Taninaka,A. Okada, M. Berthe, N. Kobayashi, O. Takeuchi andH. Shigekawa, J. Am. Chem. Soc., 2007, 129, 740.
9 X. Zhao, H. Yan, R. Zhao and W. Yang, Langmuir, 2003, 19, 809.
10 S. Kevan, Phys. Rev. Lett., 1983, 50, 526.
11 T. Sakurai, X.-D. Wang, Q. Xue, Y. Hasegawa, T. Hashizume andH. Shinohara, Prog. Surf. Sci., 1996, 51, 263.
12 C. Silien, N. Pradhan, W. Ho and P. Thiry, Phys. Rev. B: Condens.
Matter Mater. Phys., 2004, 69, 115434.
13 R. Nouchi, K. Masunari, T. Ohta, Y. Kubozono and Y. Iwasa,Phys. Rev. Lett., 2006, 97, 196101.
14 L. Piot, F. Silly, L. Tortech, Y. Nicolas, P. Blanchard, J. Roncali andD. Fichou, J. Am. Chem. Soc., 2009, 131, 12864; H. Glowatzki,B. Brker, R.-P. Blum, O. Hofmann, A. Vollmer, R. Rieger,K. Müllen, E. Zojer, J. Rabe and N. Koch, Nano Lett., 2008, 8, 3825.
15 J. Larsson, S. Elliot, J. Greer, J. Repp, G. Meyer and R. Allenspach,Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 115434.

This journal is c The Royal Society of Chemistry 2011