Published on Web 01/05/2007

Anisotropic Free-Electron-Like Dispersions and Standing Waves Realized inSelf-Assembled Monolayers of Glycine on Cu(100)Ken Kanazawa, Yasuyuki Sainoo, Yasuaki Konishi, Shoji Yoshida, Atsushi Taninaka, Arifumi Okada,
Maxime Berthe, Nobuhiko Kobayashi, Osamu Takeuchi, and Hidemi Shigekawa*
Institute of Applied Physics, CREST-JST, 21st Century COE, UniVersity of Tsukuba, Tsukuba 305-8573, Japan
Received August 31, 2006; E-mail: hidemi@ims.tsukuba.ac.jp

Knowledge and control of the self-organization of materials arethe essential foundations not only for understanding the mechanismof the phenomenon but also for the development of functional
materials and devices for practical applications. In fact, among therecent developments in nanoscale science and technology, therealization of new functions via self-organization is the basis ofmany brilliant innovations and is one of the main goals ofresearchers. Efficient use of the multifold characteristics of organic
materials plays important roles.1-12
The formation of nanostructures is achieved, in general, bycontrolling the direct and indirect interactions between building
blocks, originating from, for example, electronic and conformationalstructures, strains, and chemical reactions. Recently, a selective
supramolecular assembly of adsorbed molecules, for example, hasbeen successfully produced through the chemical modification offunctional groups.1 For the further advance of the nanostructurebased functional devices, the understanding and control of theelectronic properties of self-organized structures based on such a
modification of interactions are key factors for success.
Here, we demonstrate a two-dimensional (2D) anisotropic
electronic structure produced through the formation of a selfassembled monolayer (SAM) of glycine molecules on a Cu(100)surface. A standing wave originating from the 2D electronic
structure was visualized, for the first time, for the SAM of organic
molecules, and the anisotropic dispersion relations reflecting thestructure of the SAM were obtained.
Glycine is the simplest amino acid and does not have any activefunctional groups except for the carboxyl and amino groups, which
are common to all amino acids, and one of the fundamental
components of biological molecules such as proteins and peptides.
Glycine molecules are evaporated in a neutral form (NH2CH2COOH). When the substrate is maintained at room temperature(RT), the hydrogen atom in the carboxyl group is removed fromthe surface and glycine molecules are adsorbed in a glycinate form
(NH2CH2COO-).12,13 The two oxygen atoms in the carboxylategroup and the nitrogen atom in the amino group, which are locatedon the top sites of Cu(100), are bonded to the Cu atoms, where thecarboxyl and amino groups are negatively and positively polarized,respectively. Among the amino acids, glycine is the only moleculethat does not have chirality, but enantiomeric isomers are observedon the Cu surface depending on the directional relationship of thetwo groups in the adsorbed form, as schematically shown in Figure
1c. The adsorption properties of this system are well characterized,indicating the importance of this material for understanding andapplication of the basic mechanism of the self-organization of apolarized molecule with chirality.
In addition to sample preparation, scanning tunneling microscopy
and spectroscopy (STM/STS) measurements were performed underultrahigh vacuum conditions (base pressure < 1 × 10-8 Pa). ACu(100) clean surface was obtained after three cycles of Ar+ ion
740 9 J. AM. CHEM. SOC. 2007, 129, 740-741

Figure 1. (a) STM image obtained at 5 K (Vs ) -100 mV, It ) 1.0 nA,0.4 ML, 150 × 150 nm2). (b) Magnification of a p(2 × 4) area withmolecular arrangement. (c) Two conformations of an adsorbed glycine
molecule.

bombardment and annealing at 820 K. After the glycine source
was outgassed by heating the Al2O3 crucible at 330 K for 5 h,glycine molecules were adsorbed on the substrate, where the source
and Cu sample temperatures were maintained at 370 K and RT,respectively. Then STM/STS measurements were performed at 5 K
using a tungsten tip.
Figure 1a is an STM image observed at 5 K of the surface
prepared by the adsorption of 0.4 monolayer (ML) glycine. As
illustrated in the magnified image (Figure 1b), there is a phase witha p(2 × 4) periodicity. While the p(2 × 4) structure is unstable at
RT, isolated molecules are stable when the molecules are adsorbed
at 5 K. Therefore, the p(2 × 4) phase structure is formed duringthe cooling of the sample after molecular deposition.14 The p(2 ×4) structure consists of alternately ordered two molecular rows along
the [110] direction, which have two different molecular conformations as indicated by red (S) and black (R) arrows in Figure 1c,
showing the existence of heterochirality for this structure.
This result indicates that the p(2 × 4) phase has the pseudo-(2× 2) structure.15 As presented in Figure 1b, the STM image ofmolecular arrangement is in good agreement with that predictedby the theoretical simulation, and the molecular arrangement isshown (Figure 1b).
Figure 2a shows the I-V (inset) and its derivative spectra
obtained for a bare Cu substrate and the p(2 × 4) structure at 5 K.
The p(2 × 4) structure exhibits a sharp peak near +130 mV. In
order to investigate the origin of this signal, we measured the dI/
dV images over the area presented in Figure 1a (lock-in modulation) 8 mV, 2.74 kHz, scan speed ) 3 ms/pixel, 512 × 512 pixels).
As shown in the images in Figure 2b, a modulated local densityof states (LDOS) is observed, which has a clear bias dependence.
From the bias dependence of the autocorrelations of themodulated LDOS (Figure 3a,b), we obtained nearly free-electronlike dispersion relations, as plotted in Figure 3c. Unlike the substrate
symmetry, the dispersion relations are anisotropic, and the effective
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Figure 2. (a) I-V and STS spectra for the glycine p(2 × 4) structure and bare Cu(100) surfaces obtained at 5 K. (b) STM topographic and dI/dV images
obtained at different bias voltages over the same area (set point: Vs ) -100 mV, It ) 1 nA, 150 × 150 nm2).

The ability to control the electronic structures through chemicalmodification, for example, by changing functional groups and side
chains, is a significant improvement in the design of interactions
for producing desired electronic structures.
In summary, an anisotropic 2D electronic structure was formed
using the self-organization of glycine molecules on a Cu(100)surface. From the analysis of the standing wave, observed for thefirst time in the SAM of organic molecules, anisotropic dispersion
relations originating from the structure of molecular rows weresuccessfully obtained.

Figure 3. (a) Autocorrelations along (a) [110] and (b) [-110] directions,obtained from the 2D autocorrelation of the dI/dV images. (c) Dispersion
relations obtained from the autocorrelations in (a) and (b).

masses obtained from the dispersion relations are (0.061 ( 0.002)me for [110] and (0.61 ( 0.04)me for [-110] directions.
The observed electronic structures are different from those that
may be caused by the Cu(100) substrate, such as simple resonance.
Therefore, the observed modulated LDOS can be attributed to thestanding wave produced by the electronic states originating fromthe interaction between the glycine SAM and the Cu substrate. As
have been observed, for example, on the (111) surface of noble
metals, standing wave, the long-range modulation of LDOS,originates from the quantum interference of low-dimensional freeelectrons.16,17 However, this is the first result that demonstrates theformation of an anisotropic 2D electronic structure in the SAM ofan organic material, reflecting the characteristics of the moleculararrangement.
The 10-fold difference in the effective masses is attributed tothe anisotropy of the molecular interactions in the SAM; that is,
the interaction along the molecular rows in the [110] direction isstronger than the interaction between the molecular rows. In fact,
although it is not shown here, the p(2 × 4) arrangement exhibitedsome phase defects formed by the substitution of a molecular row
of one chirality with a molecular row of the other chirality(appearance of SSS or RRR ordering instead of SRS or RSR as inFigure 1b). This is likely because of a larger overlap of electronicstates in molecules along the [110] direction, originating from thecarboxyl or amino groups in the molecules (Figure 1b).

Acknowledgment. This work was supported by a Grant-in-Aidfor Scientific research from the Ministry of Education, Culture,Sports, Science, and Technology of Japan. We thank Ms. Rie
Yamashita, in our group, for her help in preparing this paper.
Supporting Information Available: Procedure of the standing
wave analysis. This material is available free of charge via the Internetat http://pubs.acs.org.
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