ARTICLE pubs.acs.org/JPCC
Incommensurate Crystalline phase of n-Alkane Monolayers on Graphite (0001) Osamu Endo,* Toko Horikoshi, Nobuyuki Katsumata, Keita Otani, Takumi Fujishima, Hiromichi Goto, Kazuhiro Minami, Kouki Akaike, and Hiroyuki Ozaki Department of Organic and Polymer Materials Chemistry, Faculty of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
Ryouhei Sumii and Kenta Amemiya KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
Masashi Nakamura Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan
Nobuhiro Kosugi UVSOR, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
bS Supporting Information ABSTRACT: An incommensurate crystalline phase of the (sub)monolayer of n-C36H74 lying on graphite (0001) is observed on cooling the smectic phase using normal incidence near edge X-ray absorption fine structure spectroscopy at the carbon K-edge (NI C K-NEXAFS) and scanning tunneling microscopy (STM). The orientation of the CCC plane of the all-trans alkyl chain with respect to the substrate is confirmed by the 1s f σCH*/R resonance detected by the NI C K-NEXAFS spectra at the absorption edge. Almost all molecules take the parallel (flat-on) orientation in the smectic phase, and at least half of them change to the perpendicular (edge-on) orientation in the incommensurate crystalline phase. A lamellar structure with a width corresponding to the chain length and no internal structure is observed by STM in the smectic phase, whereas that in the incommensurate crystalline phase exhibits a ladder-like structure with a periodicity of ca. 2 nm transverse to the chain direction. The periodicity is incommensurate with the substrate lattice. The molecular orientation in the ladder-like structure is related to the molecular width varying from 0.43 to 0.63 nm in the magnified STM image.
1. INTRODUCTION The self-assembly of n-alkane on inert solid surfaces is one of the fundamental phenomena that must be clarified to understand the behavior of alkyl chains, which play important roles in membranes and macromolecules.1 On a graphite basal plane, n-alkane forms a monolayer of lamellar structures with the long axis parallel to the surface, which are typical for chain molecules.2 7 The lamellar assembly of the chain molecules is useful to prepare low-dimensional organic materials; the lamellae of the conjugated alkadiyne are converted to polydiacetylene nanowires by UV light stimulation8 or the bias voltage by the tip of the scanning tunneling microscope (STM).9,10 The polydiacetylene nanowire exhibits several phases depending on the temperature and the UV irradiation time, and the r 2011 American Chemical Society
structure in these phases is strongly affected by the properties of the side alkyl groups.11 The lamellae of n-alkane are a well-defined model not only to study the nature of the alkyl groups in the macromolecules but also for drawing a molecular picture of interfacial phenomena such as adhesion and lubrication. The properties of the alkyl chains are influenced by the chain chain and chain substrate interactions, and these interactions depend on the orientation of the molecules, as characterized by the angle between the CCC Received: October 26, 2010 Revised: February 24, 2011 Published: March 14, 2011 5720
dx.doi.org/10.1021/jp1102143 | J. Phys. Chem. C 2011, 115, 5720–5725
The Journal of Physical Chemistry C
)
resonance
isolated
E/eV
I
experimental/eV
286.9
0.209
^ CH
287.5 287.0
0.243 0.141
CH
288.0
0.015
CH
288.2
0.050
tilted-on 1 molecule
CH
287.0
0.174
edge-on 1 molecule
^
287.6
0.106
flat-on 3 molecules
CH
287.2
0.082
287.2
CH
288.1
0.059
287.8
tilted-on 3 molecules
CH CH
288.3 287.4
0.013 0.035
287.8 (287.2)
edge-on 3 molecules
^
288.7
0.194
288.3
(fef)
^
288.8
0.080
288.3
flat-on 1 molecule
spectra for the n-C36H74 (sub)monolayer on graphite measured at 120 (green line), 300 (blue line), and 400 K (black line). The σ*CH/R and σ*CH/R^ resonances for n-alkane appear at 286.9 and 287.5 eV, respectively, for an isolated molecule in our calculation. (The notations and ^ represent the direction of the orbital distribution against the CCC plane.) Because the resonance distributed parallel to the surface is enhanced in the NI spectrum, the σ*CH/R (σ*CH/R^) resonance indicates the parallel (perpendicular) orientation of the CCC plane with respect to the surface. More precisely, however, these vacant orbitals are modified upon condensation, adsorption, or both on the surface to become orthogonal to the orbitals of the adjacent molecules, substrate, or both (so-called matrix effects17,18) owing to a relatively large spatial distribution with Rydberg characters (R) of the final state. In general, the condensation causes a high energy shift of the σ*CH/R resonance of the orbital directed to the adjacent molecules. The calculated resonance energy and the relative transition probabilities for representative states are summarized in Table 1. The state indicated as 3-molecules corresponds to the calculation conducted for the center molecule surrounded by two molecules placed at d = 0.464 nm. This d value was assumed from the average distance deduced from the STM results, as shown in the next section. The orientation of the surrounding molecules is the same as that of the center one, except for the results in the bottom row. (The result shown in the bottom row is for the center molecule with the edge-on orientation surrounded by the molecules with the flat-on orientation, as indicated as fef). More detailed results of the calculation as a function of the intermolecular distance d are shown in the Supporting Information (Figure S1). The resonance directed along the CH bond (σ*CH/RCH, Figure 1b, left inset) appears for the molecules with the flat-on * /R resonance due to symmetry orientation, instead of the σCH lowering at an energy of 287.2 eV, between the σ*CH/R and σ*CH/ R^ resonances with a slightly reduced intensity. Other resonances having their moments parallel to the surface appear at a higher energy (288.1 eV) for the molecule with the flat-on orientation. These transitions are almost unaffected by the surrounding molecules, and the energy is constant as the inter* /R^ molecular distance d is changed. In contrast, the σCH resonances (Figure 1b, right inset) for the molecule with the edge-on orientation are strongly influenced by the surrounding )
)
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2.1. C K-NEXAFS. Figure 1 shows the temperature-dependent NI C K-NEXAFS spectra for the (sub)monolayer of n-C36H74 on graphite (0001). In general, the direction of the vacant orbital distribution around the excited atoms is determined by analyzing the polarization dependence of the NEXAFS spectra.16 At normal incidence, the electric vector of the incident X-ray is parallel to the surface, and hence the intensity of the π* resonance of the graphite substrate is at a minimum because almost all of the graphene sheets lie parallel to the surface in the highly oriented pyrolytic graphite (HOPG), and the π* orbitals are distributed perpendicular to the * resonance of n-C36H74 is surface. Therefore, the 1s f σCH distinguished from the substrate signals at the absorption edge (Figure 1a). It should be noted that at other incidence angles, the huge π* resonance of the graphite substrate makes it difficult to * resonance of the n-C36H74 molecules. observe clearly the σCH Figure 1b shows the 1s f σ*CH region of the NI C K-NEXAFS
state
)
2. RESULTS AND DISCUSSION
Table 1. Calculated Resonance Energies E and Relative Transition Probabilities I of C K-NEXAFS for n-C4H10
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plane of the all-trans alkyl chain and the adsorbing surface. The orientation of the CCC plane determines the intermolecular spacing, d, which varies between the different lamellar phases of the n-alkane monolayer. A vapor-deposited monolayer consisting of alkane molecules with carbon number 24 or 32 undergoes a crystalline-to-smectic, followed by a smectic-to-fluid phase transition on increasing the temperature.12 In the smectic phase at around room temperature, all molecules are arranged in a lamella with a width corresponding to the chain length, although the center of mass position of each molecule fluctuates within the lamella. d is 0.464 nm, and the CCC plane is considered to be oriented parallel to the surface from this value (flat-on orientation). At the smectic-to-fluid phase transition, intermolecular and intramolecular orders are lost simultaneously.13 The lamellar lattice in the crystalline phase is uniaxially commensurate with d = 0.426 nm, which is twice the interval between the zigzag chains of graphite (0.213 nm), and the CCC plane orientation is deduced to be perpendicular (edgeon orientation) and parallel alternately.12 The vapor-deposited monolayer forms the commensurate crystalline phase at room temperature in the presence of a heptane solvent.14 The monolayer at the liquid solid interface at room temperature shows a similar structural parameter d to that of the crystalline phase, although all molecules are considered to take the edge-on orientation by STM observation.4 In the high-temperature phase of the monolayer at the liquid solid interface for an n-alkane with carbon number 28 or 32, a sliding motion along the chain axis was observed as a thermally induced disorder.15 These observations indicate that there are several processes in the smectic crystalline transition, including the change in orientation, lattice contraction, and alignment along the chain axis. It remains unknown, however, whether these processes occur simultaneously or successively. If one or more of these processes are completed at a time when others are not, then it may result in another type of crystalline form. It is important that the orientation is confirmed by an independent technique because the d value does not necessarily correspond to the orientation. In this Article, we have observed the molecular orientation using normal incidence near-edge X-ray absorption fine structure spectroscopy at the carbon K-edge (NI C K-NEXAFS) and performed real-space imaging using STM under ultrahigh vacuum (UHV) for an n-hexatriacontane (n-C36H74) (sub)monolayer on graphite (0001) on cooling a smectic phase.
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Figure 1. NI C K-NEXAFS spectra of n-C36H74 on graphite (0001) at 120 (green), 300 (blue), and 400 K (black line). (a) Wide energy region. (b) 1s f σCH * region. Inset: Distribution of the σCH * /RCH (left) and σ*CH/R^ (right) orbitals, which are detected in the NI C K-NEXAFS spectra for the flat-on and edge-on orientation, respectively.
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molecules. The resonance with a considerable intensity appears at 287.6 eV upon adsorption and shifts to 288.7 eV (288.8 eV) when the molecule is sandwiched by two other molecules with the edge-on (flat-on) orientation. The energy shifts from 288.5 (288.6 eV) to 289.0 eV (289.2 eV) as the value of d changes from 0.52 to 0.42 nm. It should be noted that the intensity is greatly reduced when the surrounding molecules take the flat-on orientation. The broad band centered at 287.5 eV in the 400 K spectrum corresponds to the convolution of the σ*CH/R and the σ*CH/R^ resonances. This assignment indicates that a large number of methylene sequences are in a random orientation against the surface. As a result of the melting transition, both the intermolecular and the intramolecular orders disappear,13 and considerable gauche conformations are induced in the molecules in the fluid phase. The absorption edge is at a lower energy than the other two spectra, and this is consistent with the fact that alkyl chains are separated from each other in this phase and hence less influenced by the neighboring molecules. The bands located at 287.2 eV in Figure 1b at 300 K and * /RCH resonance for the reduced at 120 K are assigned to the σCH molecules with the flat-on orientation. In contrast, the relatively broad band at 288.3 eV observed in the 120 K spectrum is attributed to the σCH * /R^ resonance. The band at 287.8 eV * /RCH observed in the 300 K spectrum is assigned to the σCH resonance, which appears at 288.1 eV in the calculation. The contribution of the σ*CH/R^ resonance to this band is estimated to be small because the d dependence of the calculated resonance energy indicates that the neighboring molecules should by placed at least 0.6 nm apart from the center molecule with the edge-on orientation to exhibit the σCH * /R^ resonance at ∼288 eV. The distance is much larger for n-C36H74 at 300 K in the smectic phase. The assignment indicates that the molecules take the flat-on orientation at 300 K, and at least half of the molecules change their orientation to become edge-on at 120 K. It is noted that in the tilted-on orientation, which is an intermediate between the
Figure 2. Temperature-dependent STM images of a n-C36H74 monolayer on graphite (0001). (a) Smectic phase observed at T = 300 K with sample bias voltage V= 2.0 V and tunneling current I = 13 pA. Image size is 140 nm 140 nm. (b) Incommensurate crystalline phase; T = 80 K, V = 2.0 V, I = 60 pA, 100 nm 100 nm. (c) Smectic phase; T = 260 K, V = 2.0 V, I = 30 pA, 40 nm 40 nm. (d) Incommensurate crystalline phase; T = 80 K, V = 2.0 V, I = 60 pA, 40 nm 40 nm. Black lines are guides for the lamellar direction.
* /RCH resonance similar flat-on and edge-on orientations, a σCH to that in the flat-on orientation appears but is reduced upon condensation to be almost unobservable in a spectrum. The orientation change occurs at temperatures
