Surface Science 632 (2015) L1–L4

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Surface Science Letters

Variation in anisotropic dispersion relations of self-assembled monolayer
on Cu(001) induced by modulation of molecular structures
Ken Kanazawa a, Miki Nakamura a, Hui Huang b, Atsushi Taninaka a, Hidemi Shigekawa a,⁎ab

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
Hefei University, Chinaa r t i c l e

i n f oArticle history:
Received 19 May 2014
Accepted 16 July 2014
Available online 31 July 2014
Keywords:
Scanning tunneling microscopy
Anisotropic dispersion relation
β-Alanine
Amino acid2DEG
Cu(001)a b s t r a c t
The two-dimensional electron gas (2DEG) states formed by self-assembled monolayers (SAM) of β-alanine molecules on a Cu(001) surface showed anisotropic dispersion relations different from those formed by glycine SAM.
β-Alanine has a structure with an additional methylene group compared with glycine, and enantiomeric isomers
were formed through adsorption, similarly to glycine. The anisotropic ratio of the effective masses was changed
from 10 for glycine to 3.6 for β-alanine, suggesting the possibility of manipulating electronic structures by modiﬁcation of the molecular structures. Although the growth modes were different for β-alanine and glycine, 2DEG
states with standing waves were observed only for the p(2 × 4) phase in both cases, suggesting a key role of theinteractions in the arrangement of this phase, together with the importance of the enantiomeric isomers formed
through adsorption, which is also a characteristic of both molecules.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
The control of low-dimensional electronic structures has been a key
factor in the development of electronic devices. The modulation of freeelectron-like states arising in organic thin ﬁlm/metal interfaces has
been attracting considerable attention because of its potential for further device applications as well as its basic importance for understanding fundamental mechanisms in molecular electronics [1–11]. The
quantum conﬁnement of surface electrons is an interesting research target, and various types of molecules have been used to produce quantum
corrals with different shapes and sizes [1–6]. On the other hand, thetwo-dimensional electron gas (2DEG) states associated with a selfassembled monolayer (SAM) of molecules on a metal surface are also
the area of study attracting interest [7–11]. Recent scanning tunnelingmicroscopy/spectroscopy (STM/STS) studies have revealed the formation of free-electron-like structures, for example, in the SAMs of
3,3,9,10-perylenetetracarboxylic-acid-dianhydride molecules on
Ag(111) [7] and glycine molecules on Cu(001) [8]. In the glycine/Cu(001) structure, a dispersion relation with tenfold anisotropy,
which is different from the symmetry of the substrate, was observed[8]. Exploring the possibility of manipulating such 2DEG structures isdesirable for advancing the development of molecular electronic
devices.

⁎ Corresponding author. Tel./fax: +29 853 5276.
E-mail address: hidemi@ims.tsukuba.ac.jp (H. Shigekawa).
URL: http://dora.bk.tsukuba.ac.jp (H. Shigekawa).

http://dx.doi.org/10.1016/j.susc.2014.07.022
0039-6028/© 2014 Elsevier B.V. All rights reserved.

In this paper, we present the ﬁrst results for the variation in the anisotropic dispersion relations in SAMs on a Cu(001) surface induced by
the modulation of molecular structures.
β-Alanine was chosen as the sample because of its characteristic property of chirality similar to that of glycine. Among the α-amino acids, glycine is the only molecule that does not have chirality (Fig. 1(a)). However,enantiomeric isomers of glycine appear on a Cu surface through the dissociation of hydrogen, depending on the directional relationship of the twogroups in the adsorbed form as schematically illustrated in Fig. 1 (c) [12].
Two different structures, p(2 × 4) and c(2 × 4), are formed with different
arrangements of the isomers, i.e., the two isomers are alternately ordered
for the p(2 × 4) phase (Fig. 1(d)), while the c(2 × 4) phase consists of only
one type (Fig. 1 in ref. [13]). The 2D electronic structure with tenfold anisotropy of effective mass was observed only for the p(2 × 4) phase, suggesting the importance of this arrangement.
Alanine has two isomers, i.e., α- and β-alanine. α-Alanine naturally has R- or S-chirality (D - or L -alanine), while β-alanine has astructure with an additional methylene group (\CH2\) in its main
chain compared with glycine and does not have chirality, as shownin Fig. 1 (b). In the case of α-alanine, even molecules with both chiralities are adsorbed together, only c(2 × 4) domains consisting of
D- or L-alanine are separately grown, and the p(2 × 4) phase, which
consists of the molecules with two isomers and has a 2DEG structurein the case of glycine, is not formed [13–15]. This result suggests theimportance of the process of producing enantiomeric isomers
through adsorption on a Cu(001) surface, as illustrated for the caseof glycine. Namely, if β-alanine molecules produce enantiomeric isomers through adsorption similar to the case of glycine, the formation

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molecules were adsorbed on the substrate where the source and Cu
sample temperatures were maintained at 370 K and room temperature(RT), respectively. Then STM/STS measurements were performed at 5 K
using a tungsten tip.

3. Results and discussion

Fig. 1. Schematic structures of (a) glycine and (b) β-alanine molecular structures,(c) chirality of glycine molecule produced by adsorption on a Cu(001) surface, and(d) molecular arrangement of the p(2 × 4) phase.

of the p(2 × 4) phase and the production of 2DEG states may be realized. Furthermore, the existence of a methylene group may modify
the electronic structure.
2. Materials and methods
A clean Cu(001) surface was prepared by three cycles of Ar+ ion
bombardment and annealing at 820 K. After β-alanine source wasoutgassed by heating the Al2O3 crucible at 330 K for 5 h, β-alanine

Fig. 2 (a) to (c) shows STM images obtained in the growth process ofa β-alanine molecular SAM on a Cu(001) substrate. Three structures
were observed in the growth step with increasing amount of adsorbed
molecules: (1) a disordered phase, (2) a stripe phase and (3) a p(2 × 4)
phase. With increasing amount of adsorption, the disordered phase appeared ﬁrst, and then the stripe phase emerged (Fig. 2(a)). After the
stripe phase covered the surface (Fig. 2(b)), the p(2 × 4) phase, which
was observed for glycine, was formed (Fig. 2(c)).
Fig. 2 (d) and (e) shows magniﬁed STM images of the (a) disorderedphase and (b) stripe phase. In the disordered phase, β-alanine moleculestended to form linear structures in the [010] and [100] directions. The
stripe phase appears to be linearly ordered in the [010] direction asshown in Fig. 2 (d); however, this is due to the alternate variations in contrast as shown in the magniﬁed image (Fig. 2(e)). The high-resolution
image shows that the stripe phase is formed by a 4√2 × 3√2-R45°
structure.
Fig. 3 (a) shows a magniﬁed STM image of the p(2 × 4) . Although it is
difﬁcult to determine the detailed structures from the STM image because the characteristics of the molecular images are less clear thanthose of the case of glycine [8], the structural model was considered byassuming arrangement similar to that of glycine shown in Fig. 1 (d). To
examine the structure, we carried out a simple density functional theory
calculation using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) with the ABINIT code with plane-wave-based
norm-conserving pseudopotentials [16,17]. The (2 × 4 × 4) supercell including the slab model with three Cu(001) layers and two β-alaninates

Fig. 2. STM images of β-alanine molecular structures with increasing the amount of adsorbed molecules ((a) Vs = −250 mV, It = −1 nA, (b) Vs = −350 mV, It = −1 nA, (c) Vs = 350 mV,
It = 0.3 nA). Magniﬁed STM images of (d) disordered and (e) stripe structures (thermal drift, corrected).

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Fig. 3. (a) Magniﬁed image of the p(2 × 4) structure in Fig. 2 (Vs = −300 mV, It = −1 nA). (b) Schematic of slab models used for calculation. (c) Simulated image calculated using themolecular structure shown in (b). (d) dI/dV curves obtained for a bare Cu surface and above the stripe and p(2 × 4) structures.

shown in Fig. 3 (b) was used for the calculation. The energy cutoff of thecalculation was 60 Ry. The k-point meshes of 4 × 4 × 1 were used for the
calculations. From the spatial distribution of the local density of states(LDOS) near the Fermi energy EF in the surface region (EF-0.5 eV b Eb EF), we obtained the result shown in Fig. 3 (c), which is comparable
to the STM image shown in Fig. 3 (a) [18]. The agreement of the experimental and simulated images is good, indicating the validity of thestructural model for the p(2 × 4) phase. Namely, although the growthmode of β-alanine is different from that of glycine [13], a p(2 × 4)
phase with the molecular arrangement similar to that of glycine isformed.
Next, we investigated the electronic structure of the p(2 × 4) phase.
Fig. 3 (d) shows dI/dV curves measured above the stripe and p(2 × 4)
phases. The p(2 × 4) phase exhibits a step-like shape, suggesting the existence of a 2DEG structure similar to that appearing in the case of glycine molecules [8]. To examine the origin of this signal, we measured

Fig. 4. (a) Topographic image and (b) to (f) typical dI/dV images obtained over the area in (a). (g) Dispersion relations obtained from the bias dependence of the wavelength in the dI/dV
images. The results obtained for the case of glycine in a previous paper [8] are also shown.

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Table 1
Values of effective masses m* in the [110] and [110] directions (me: electron mass) and theenergy minima E0 of the dispersion relations obtained for β-alanine and glycine [8].
β-Alanine

Glycine−2

m*[110]/mem½110 /me

6.1 × 10
6.1 × 10−1

E0(meV)~110

8.7 × 10−2
3.1 × 10−1

the dI/dV images. Fig. 4 shows (a) the topographic image and (b) to(f) dI/dV images obtained over the area in (a) (lock-in modulation =8 mV, frequency = 2.74 kHz, scan speed = 2 ms/pixel, 512 × 512 pixels).
The modulated LDOS of the 2DEG were clearly observed as expected,
which are similar to the case of glycine [8]. From the bias dependenceof the autocorrelations of the modulated LDOS, nearly free-electronlike dispersion relations were obtained, which is shown in Fig. 4(g) together with the results obtained for glycine in the previous work.
The effective masses obtained for β-alanine are 8.7 × 10−2 me and3.1 × 10−1 me (me: electron mass) for the [110] and [110] directions,which are more isotropic than those obtained for glycine, 6.1 × 10−2
me and 6.1 × 10−1 me for the [110] and [110] directions, respectively.
The same value of the energy minimum E0 in both cases is considered
to be due to the fact that the methylene group does not shift the energy
level markedly, probably because of its electroneutrality. The obtained
values are summarized in Table 1.
The tenfold difference in the effective masses for the case of glycine
is attributed to the anisotropy of the molecular interactions in the SAM;
that is, the interaction along the molecular rows in the [110] direction isstronger than that between the molecular rows. In fact, the p(2 × 4) arrangement exhibited some phase defects formed by the substitution ofa molecular row of one chirality with a molecular row of the other chirality. This is likely to be because of the larger overlap of electronic statesin molecules along the [110] direction, originating from the carboxyl oramino groups in the molecules (Fig. 1(d)).
In consideration of the similar p(2 × 4) arrangement for both molecules, shown in Figs. 1 (d) and 3 (c), the rather isotropic electronic structure realized by β-alanine may be a result of the effects of the direct
modiﬁcation of intramolecular orbitals and strain, which may induce
the atomic displacement of the amino and carboxylate groups fromthe case of glycine, caused by the additional methylene group in the
β-alanine molecule. To obtain a further understanding, however, moredetailed analysis of the molecular interactions is necessary, which isleft for a future study.
4. Conclusions
In conclusion, the possibility of manipulating the dispersion relations of 2DEG structures formed in a SAM/metal interface wasattempted by comparing the electronic structures obtained using βalanine molecules and glycine molecules. Although the only difference
between the structures of β-alanine and glycine is an additional methylene group (\CH2\) in β-alanine, the anisotropic ratio of the effective
masses was successfully changed from 10 for glycine to 3.6 for alanine.
Although a three-step growth process was observed for β-alanine,
2DEG states were only produced for the p(2 × 4) phase, similar to thecase of glycine, suggesting a key role of the interactions in the arrangement of this phase, together with the importance of the enantiomericisomers appearing through adsorption on the Cu surface, observed forboth molecules. The manipulation of anisotropy in free-electron-like
2DEG states by the modiﬁcation of the molecular structures to provide
additional functions may be a possible means of further developing future molecular devices.

Acknowledgments
Support from Japan Society for the Promotion of Science (Grants-inAid for Scientiﬁc Research) is acknowledged.

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