ARTICLE pubs.acs.org/JPCC
Molecular-Scale Structure of a Nitrobenzene Monolayer on Si(001) Guowen Peng,† Soonjoo Seo,‡ Rose E. Ruther,§ Robert J. Hamers,§ Manos Mavrikakis,*,‡ and Paul G. Evans*,‡,þ †
Department of Chemical and Biological Engineering, ‡Materials Science Program, §Department of Chemistry, and Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
þ
ABSTRACT: Nitrobenzene molecules can be attached to Si(001) by exposing the Si(001) (2 1) surface to nitrobenzene gas at room temperature. The resulting monolayer lacks longrange order in scanning tunneling microscopy images but shows signs of a local periodic arrangement of molecules. Selfconsistent plane-wave density functional theory calculations find that the energy gained per molecule in the adsorption of nitrobenzene is nearly constant as a function of nitrobenzene coverage and that it is energetically favorable to form coverages as high as one molecule per Si dimer. Ab initio molecular dynamics (AIMD) simulations show that the migration of oxygen transforms an initial configuration in which the nitrobenzene molecule bridges a Si dimer into lower energy configurations with oxygen separated from the nitrobenzene phenyl group. A previously unknown low-energy configuration, characteristic of SiO2 bonding, is identified by the AIMD calculations. The energy barriers for O migration into Si backbonds are calculated by using the climbing-image nudged elastic band method. The calculated dipole moments of nitrobenzene on Si(001) varied from 0.11 to 0.45 D, depending on the molecular configuration. Nitrogen is observed using X-ray photoelectron spectroscopy in a concentration consistent with the attachment of one nitrobenzene molecule per Si dimer.
1. INTRODUCTION Interfaces between organic semiconductors or in hybrid organic semiconductor/inorganic semiconductor composites have crucial roles in determining the electronic properties of devices ranging from field-effect transistors (FETs) to photovoltaics. The thickness of the charge accumulation layer in an organic FET, for example, is on the order of the molecular size, effectively making these devices completely dependent on the few molecular layers at the gate insulator/semiconductor interface.1 Molecular monolayers at interfaces can provide specific electronic properties, including dipole moments, and yield dramatic changes in device current-voltage charateristics.2 The electrical polarization of a CF3-based self-assembled monolayer at the gate insulator/ semiconductor interface of FETs, for example, shifts the threshold voltage by tens of volts.3 Inserting such a dipole moment at a metal/ organic semiconductor interface likewise modifies the Schottky barrier.4,5 In addition to dipole moments, interface layers can provide other specific electronic properties. Monolayers incorporating C60 or azobenzene groups have interface acceptor states or reconfigurable dipoles that produce effects of similar magnitude to the threshold voltage shift produced by a static dipole.6,7 In a structural sense, the addition of an organic monolayer to an inorganic semiconductor can modify the chemical interaction of the substrate with a subsequently deposited organic semiconductor layer, allowing the growth of large crystallites of organic layers on silicon8 and providing a surface sufficiently smooth that structural studies can be performed using scanning tunneling microscopy (STM).9,10 r 2011 American Chemical Society
Organic/inorganic interfaces can be created and characterized in ultrahigh vacuum (UHV) with strict control over the monolayer composition and the inorganic surface structure. Silicon surfaces provide a crystallographically perfect and chemically pure starting point, and are thus the ideal model system for the atomic-scale study of these layers. Examples of interfaces created under UHV conditions include the attachment of cyclopentene,11 styrene,12 butadiene,13 nitroethane,14 and nitrobenzene15,16 to Si(001). The silicon-nitrobenzene system is particularly interesting because the free nitrobenzene molecule (C6H5NO2), consisting of a phenyl ring and NO2 group, has a gasphase dipole moment of several Debye. An ordered array of molecules possessing even a fraction of this large dipole moment would be sufficient to induce significant band offsets at Si/ organic electronic interfaces, providing the means to influence transport in either Si or an organic overlayer. The possibility that the nitrobenzene/Si structure may produce significant electronic effects has motivated us to perform a detailed study of its structure and formation. Previous investigations of nitrobenzene adsorption on Si(001) have used Auger electron spectroscopy and infrared absorption spectroscopy to probe the formation of the nitrobenzene/Si interface.16,17 Among the results of these studies are observations Received: July 26, 2010 Revised: January 5, 2011 Published: February 1, 2011 3011
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The Journal of Physical Chemistry C that the nitrobenzene is in fact attached to the Si surface after functionalization and that the nitrobenzene/Si interface is stable at elevated temperatures. No measurable change in surface carbon concentration was detected during heating from room temperature to 1000 K.17 Cluster-model calculations of the structural properties of nitrobenzene on Si(001) have shown that the nitro group of nitrobenzene reacts with Si(001) in two stages.16,17 First, the nitrobenzene molecule attaches to a dimer on the Si (001) 2 1 surface to create a configuration in which the two oxygen atoms of nitrobenzene are each attached to Si atoms in the same dimer. The initial arrangement is rapidly converted to more stable configurations in which the oxygen atoms are each bonded to Si atoms and the remaining fraction of the nitrobenzene molecule is bonded through N to a single surface Si atom.17 Here, we present a combined experimental and theoretical study of the molecular-scale structure and electronic properties of the nitrobenzene/Si (001) interface. We have used STM to examine the structure of a layer of nitrobenzene molecules on Si(001), probing the uniformity of the molecular structure across large areas. The results of the surface characterization described in detail below show that, in comparison with other molecular layers such as cyclopentene,11 nitrobenzene does not form functionalized silicon surfaces with long-range order. Experimental results are compared with theoretical predictions derived from density functional theory (DFT) and ab initio molecular dynamics (AIMD). AIMD results show that a number of configurations of nitrobenzene molecules and fragments of these molecules can be kinetically reached on the Si(001) surface by the migration of O from nitrobenzene to the Si substrate. The AIMD study also identifies a new low-energy structure of the nitrobenzene/Si surface that is distinct from the structures that have been previously considered. The dipole moment of the nitrobenzene/Si(001) interface is dramatically reduced compared to the dipole moment of the nitrobenzene molecule in the gas phase. The remaining ordered moment, however, may still be sufficient to affect transport in subsequent layers of organic semiconductors.
2. METHODS 2.1. Experimental Section. Structural studies of the nitrobenzene/Si(001) surface were performed using UHV STM. Phosphorus-doped n-type Si(001) samples with 0.07-0.1 Ω cm resistivity were cleaned using three cycles of the Interuniversity Microelectronics Center (IMEC) process.18 Samples were oriented to expose the (001) surface with an accuracy of (0.5°. After transfer into UHV, Si(001) (2 1) surfaces were prepared by degassing the samples at 600 °C overnight and then heating to 1250 °C for 5 s. The samples were cooled rapidly to ∼1000 °C, cooled to ∼500 at 1 °C/s on the heating stage, and finally slowly cooled to room temperature. This procedure yields a Si(001) surface exhibiting the (2 1) reconstruction, as shown in Figure 1a, which serves as the starting point for subsequent functionalization. Images of the Si(001) (2 1) surface were acquired with a tip bias of -1.5 V and a tunneling current of 0.5 nA. Nitrobenzene vapor was introduced into the UHV chamber from the gas above room-temperature nitrobenzene liquid using a leak valve. The liquid was purified by several freeze-pumpthaw cycles to remove dissolved gases. The Si sample was exposed to the vapor for 300 s at room temperature at an indicated pressure of 1 10-7 Torr. STM images after exposure
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Figure 1. STM images before and after exposure of a Si(001) surface to nitrobenzene. (a) Si(001) (2 1) surface prior to exposure to nitrobenzene. (b) After exposure to 30 Langmuir of nitrobenzene. The inset arrow in part b indicates the direction of the rows of nitrobenzene molecules, which form perpendicular to the substrate dimer rows.
to nitrobenzene were acquired with a bias voltage of -1.9 V and a tunneling current of 250 pA. The coverage of nitrogen resulting from the addition of nitrobenzene to the surface was probed using X-ray photoelectron spectroscopy (XPS) after moving the Si sample from the UHV STM chamber to a separate UHV system. We deposited 1 molecular monolayer of pentacene, with a thickness of 1.5 nm, onto the sample before the transfer in air between the UHV chambers. XPS spectra showed a single nitrogen photoelectron peak at a binding energy of 396.5 eV, corresponding to photoemission from the N (1s) state. The area concentration of nitrogen atoms, σN, was calculated from the XPS spectra using19 σN ¼
AAN SSi et=ðλN sinðθÞÞ FSi λSi, Si sinðθÞ t=ðλ sinðθÞÞ ASi SN e Si
ð1Þ
Here FSi is the number of Si atoms per unit volume in the Si substrate, t is the combined thickness of the nitrobenzene layer above the N atoms and the pentacene capping layer, and θ = 45° is the takeoff angle with respect to the plane of the surface. The escape depths λN and λSi apply to electrons emitted from N and Si through the capping layer and λSi,Si is the escape depth for electrons emitted from Si through Si. For Si, we used λSi, Si = 31.6 Å.20 The electron escape depth λ for electrons in the nitrobenzene and capping layer was estimated using the expression given by Laibinis et al. for alkanethiol monolayers, in which λ varies with the electron energy E according to λ = 9.0 Å þ (0.022 Å/eV)E.21 With this approximation, the escape depths for N and Si photoelectrons in the molecular layer are λN = 33 Å and λSi = 40 Å. AN and ASi are the experimentally measured intensities of photoemission from N and Si. SSi and SN are the atomic sensitivity factors for Si and N.22 The total thickness t of the molecular layers above the interface was taken to be 20 Å, but because both λN and λSi are larger than t, the resulting coverage is insensitive to the precise choice of t. The surface concentration of nitrogen atoms derived using eq 1 is 4.4 1014 cm-2. The experimental uncertainty in the N coverage is (0.8 1014 cm-2, arising from the uncertainty in the measurement of the intensity of the N (1s) peak. The N coverage is thus, within experimental uncertainty, consistent with the attachment of one N atom to each Si(001) dimer (which have a surface concentration of 3.4 1014 cm-2). 2.2. Theoretical. The DFT and AIMD studies were performed using the Vienna ab initio Simulation Package (VASP).23,24 Ultrasoft pseudopotentials25 were used to describe 3012
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The Journal of Physical Chemistry C the electron ion interactions. The exchange correlation functional was described by the generalized gradient approximation (GGA-PW91).26 The electron wave function was expanded using plane waves with an energy cutoff of 400 eV. The Si(001) sample was modeled with a slab geometry consisting of ten layers of Si atoms. A vacuum region of ∼20 Å was used to separate periodically repeated slabs. The dangling bonds of Si atoms at the bottom of the slab were saturated with hydrogen atoms to mimic the bulk. The relaxed Si lattice constant in these calculations is 5.46 Å, close to the experimental value of 5.43 Å. To study the adsorption of nitrobenzene on Si(001) at various surface coverages, the simulations were repeated with one, two, three, and four nitrobenzene molecules in a p(4 2) unit cell, corresponding to coverages of 1/4, 1/2, 3/4, and 1 ML, respectively. Here, 1 ML is defined as one nitrobenzene molecule per Si dimer. The Brillouin zone was sampled by a Monkhorst-Pack grid corresponding to 8 8 1 reciprocal space points for a (1 1) surface unit cell.27 All atoms in the slab, except for those in the bottom two Si layers and the H atoms, were fully relaxed to Hellmann-Feynman forces of less than 0.05 eV/Å. The dipole correction was included and the electrostatic potential was adjusted accordingly.28 The binding energy Eb is defined as the total energy difference between the adsorbed system and the sum of the total energies of the clean Si surface and gas-phase nitrobenzene. To examine how O atoms migrate from the nitrobenzene molecule to Si and to search for other stable structures, we performed AIMD simulated annealing studies29,30 on models with 1 ML of nitrobenzene adsorbed on the Si(001) p(2 1) surface. The model was first equilibrated at temperatures up to 1500 K for 4 ps and then quenched to 300 K in 4 ps, followed by a static geometry optimization of the final structure. A time step of 2 fs was used, and 2000-step MD simulations were run. To gain more detailed information on the kinetics of the structural evolution, the reaction pathways between different models obtained from the simulated annealing calculations were further investigated using the climbing-image nudged elastic band method (CI-NEB).31
3. RESULTS AND DISCUSSION 3.1. STM Images. The sample was imaged using STM to examine the structure of the nitrobenzene/Si(001) surface. The STM image in Figure 1b shows the Si(001) surface after exposure to 30 Langmuir of nitrobenzene. An STM-based estimate of the nitrobenzene coverage was derived by counting molecules within a defined area of two STM images and dividing by the number of dimers occupying that area. Nitrobenzene molecules occupy 86% of the dimer sites, indicating that nitrobenzene reaches nearly one monolayer coverage on Si(001). The high coverage of nitrobenzene molecules on Si(001) observed using STM is consistent with the XPS spectra described in section 2.1. Only a limited, local, degree of ordering is visible in the spatial distribution of the nitrobenzene molecules shown in the images of Figure 1b. Although rows of molecules along dimers are clearly visible in the images, there are no large areas of ordered features. Figure 2a shows an enlarged section of the larger image appearing in Figure 1b, in which rows of molecules extend along dimer rows from the top left of the image toward the bottom right. The cross section plotted in Figure 2b, along a line perpendicular to the dimer rows of Figure 2a, shows that the rows are periodically spaced. The separation between the rows is, within the experimental accuracy of the STM measurements,
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Figure 2. (a) Enlarged area of the STM image in Figure 1b showing the local ordering of adsorbed nitrobenzene molecules on Si(001). (b) Height profile along the white line of the image in part (a). The dashed vertical lines are separated by 8.8 Å, which is within the experimental error of the spacing between Si dimer rows. (c) STM image of the Si(001) surface after exposure to 30 L of nitrobenzene, in which monatomic silicon steps are visible at positions marked with arrows.
equal to the spacing of Si dimer rows on Si(001). The arrangement of the bright molecular features in Figure 2 is consistent with a structure in which nitrobenzene molecules are attached to Si dimers of the Si(001) surface and locally ordered as a result of the dimer periodicity. The periodicity of the molecular rows of nitrobenzene is observed only over a short distance, indicating that there are multiple configurations possible for the attachment of nitrobenzene on Si(001). At a larger scale, the height difference associated with single atomic steps on Si(001) is translated through the nitrobenzene layer. Despite the disorder, the surface remains sufficiently smooth so that steps can be detected, as shown in Figure 2c. 3.2. DFT and MD Calculations. A. Nitrobenzene on Si(001). We first studied the binding of nitrobenzene on Si(001) at a low coverage, using one nitrobenzene molecule in a p(4 2) unit cell, equivalent to a nitrobenzene coverage of 1/4 ML. Two different 1/4 ML structures were studied in the DFT calculations. First, the NO2 configuration, in which the nitro group bridges a single Si dimer, has the optimized structure shown in Figure 3a. The binding energy of nitrobenzene in the NO2 configuration is -2.53 eV, close to the result of cluster calculations reported by Mendez de Leo and Teplyakov.16 In comparison to a free nitrobenzene molecule, the N-O bond length in the NO2 adsorbed configuration is increased from 1.24 to 1.49 Å. The respective Si-O bond lengths are both 1.71 Å. A second configuration, in which the nitrobenzene molecule is attached to two adjacent Si dimers, was also considered. The fully optimized structure for this two-dimer configuration, shown in Figure 3b, has a binding energy of -2.19 eV. This structure is energetically less favorable, by 0.34 eV, than the structure with the NO2 group bonding to a single Si dimer and, hence, will not be discussed further. We also examined the dependence of the binding energy on the nitrobenzene surface coverage by calculating two, three, and 3013
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Figure 3. Low-coverage (1/4 ML) relaxed geometries of nitrobenzene on Si(001) in the p(4 2) geometry. (a) Nitrobenzene binding to a single Si dimer in the NO2 configuration. (b) Nitrobenzene binding to two adjacent Si dimers. The NO2 structure in (a) is 0.34 eV more stable than structure (b).
four nitrobenzene molecules adsorbed on the Si(001) p(4 2) unit cell in the NO2 configuration, corresponding to a nitrobenzene coverage of 1/2, 3/4, and 1 ML, respectively. The calculated average binding energy per nitrobenzene is -2.53, -2.52, -2.48, and -2.46 eV for the coverage of 1/4, 1/2, 3/4, and 1 ML, respectively. The calculated differential binding energy of the second, third, and fourth nitrobenzene molecule is -2.51, -2.41, and -2.38 eV, respectively. These results clearly indicate that it is energetically favorable to reach a 1 ML of nitrobenzene on Si(001). B. MD Simulated Annealing of O Migration. The NO2 configuration described in the previous section subsequently transforms to other more stable structures. Previous studies have shown that the O atoms of a surface nitro group can migrate into Si backbonds by breaking the relatively weak N-O bonds and forming stronger Si-O bonds.16,17,32 To investigate how O migrates from the nitrobenzene molecule to the Si surface and to search for other stable structures, we performed AIMD simulated annealing studies starting from the NO2 configuration at a coverage of 1 ML in a Si(001) p(2 1) unit cell. Two sets of MD simulated annealing runs were conducted using different temperatures. In the first set of MD simulated annealing studies, we equilibrated the NO2 configuration at 1000 K for 4 ps. No new stable structure was found during this annealing process, indicating that the NO2 configuration is quite stable. After the NO2 configuration was quenched from 1000 to 300 K for 4 ps, we observed the migration of one O atom into a Si backbond, yielding a configuration we term NOSi. The potential energy of the structure during this 2000-step MD quenching process is shown in Figure 4. The statically optimized geometries of the initial NO2 and final NOSi structures are shown as insets (i) and (ii) in Figure 4. In the NOSi configuration, the nitrobenzene phenyl ring is titled by 19° with respect to the surface normal and the Si-O bond lengths are 1.65 and 1.70 Å, slightly shorter than the Si-O bond on the surface (1.73 Å). The binding energy calculated for the NOSi structure is -4.62 eV, indicating that
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Figure 4. Potential energy profile of the adsorbed structure of nitrobenzene at 1 ML coverage during quenching of the NO2 configuration from 1000 to 300 K for 4 ps with a time step of 2 fs. Inset: Models of the (i) NO2 and (ii) NOSi configurations.
Figure 5. Potential energy profile during quenching of the NOSi configuration at 1 ML nitrobenzene coverage from 1500 to 300 K for 4 ps with a time step of 2 fs. Inset: Models of the statically relaxed initial (i) NOSi structure and the final (ii) NSi2-pre structure. The more stable NSi2 and NSi2-b configurations are shown in insets (iii) and (iv), respectively. The value in eV below each structure indicates its binding energy.
this structure is more stable than the NO2 structure (binding energy -2.76 eV). To examine whether the second O atom of the nitro group in the NOSi configuration would migrate into a Si backbond to potentially further stabilize the NOSi structure, we quenched the NOSi from 1500 to 300 K for 4 ps. The potential energy profile of this quenching process is shown in Figure 5. The N-O bond was broken during quenching and the second O atom moved to a Si backbond, resulting in the decrease in the potential energy shown in Figure 5. The starting NOSi structure is shown as inset (i) in Figure 5. The final structure, labeled NSi2-pre in inset (ii) of Figure 5, has a binding energy of -6.38 eV, 1.76 eV more stable than NOSi. The configuration can be further stabilized slightly by moving the 3-fold coordinated O atom upward so that it bonds 3014
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Figure 6. Potential energy profile during quenching of the NO2 configuration of nitrobenzene on Si(001) at 1 ML coverage from 1500 to 300 K for 4 ps with a time step of 2 fs. See Figure 7 for detailed structural information corresponding to this profile. Figure 8. Potential energy surface describing the structural evolution of nitrobenzene adsorbed on Si(001) at 1 ML coverage. Numbers in blue give the relative heights of the topmost H of the phenyl ring referenced to the height of that atom in the NO2 configuration. Numbers in black and red give the activation energy barrier in eV for the transformations between configurations shown in the insets.
Figure 7. Oxygen migration during molecular dynamics simulated annealing of the NO2 configuration, shown in (a), from 1500 to 300 K for 4 ps. The potential energy profile corresponding to this process is shown in Figure 6. (b-e) Statically relaxed versions of the structures identified during MD at 204, 492, 1586, and 2086 fs, respectively. Values in electronvolts below the molecular structures give the binding energy of each statically relaxed structure at a nitrobenzene coverage of 1 ML on Si(001).
with only two Si atoms, forming the NSi2 configuration shown in inset (iii) of Figure 5. The NSi2 configuration has a binding energy of -6.71 eV. An alternative structure similar to the NSi2 configuration can generated by rearranging the two O atoms so that one O atom lies on either side of the plane of the phenyl ring, as shown in inset (iv) of Figure 5. This structure, the NSi2-b configuration, has a binding energy of -6.40 eV. In a second set of MD simulated annealing studies, we equilibrated the NO2 configuration at a higher temperature of 1500 K and then quenched the system from 1500 to 300 K. Different routes for O migration were observed in quenching from 1500 to 300 K than were observed while quenching from 1000 to 300 K. Figure 6 shows the potential energy profile of the structure during the quenching process. Snapshots of the migration of the O atom in selected steps are shown in Figure 7. The initial NO2 configuration is shown in Figure 7a. After 204 fs, one N-O bond is broken and the O atom reacts with atoms in a Si dimer to form three Si-O bonds, which increases the interaction strength by 1.18 eV, Figure 7b. A N-Si bond is then formed at 492 fs, as shown in Figure 7c, stabilizing the structure further by 0.87 eV. The second N-O bond breaking and O atom migration occurred at 1586 fs, shown in Figure 7d, stabilizing the structure by a further 1.56 eV. Finally, a second N-Si bond is formed at
approximately 2 ps and the structure relaxes to the most stable geometry shown in Figure 7e. In the final structure reached during quenching from 1500 to 300 K, which we term NSi2-SiO2 and show in Figure 7(e), the Si atom in the second N-Si bond comes from the subsurface. Three Si atoms reach a configuration on the p(2 1) surface in which all of the O atoms are nearly at the same height. Two Si atoms that have one bond each to O and N. The third Si atom bonds to both O atoms. The SiO2-type bonding in the NSi2-SiO2 configuration significantly enhances the stability of the structure. The calculated binding energy of this structure is -7.72 eV, lower than that of the NSi2 configuration in inset (iii) of Figure 5 by 1 eV. The NSi2-SiO2 configuration is significantly more stable than the structures reported in previous studies. To the best of our knowledge, this structure is identified here for the first time. C. Reaction Pathways of Structural Evolution. To examine the kinetics of O migration discussed above, we calculated the energetics of the reaction pathways between different configurations and estimated the reaction activation energy barriers using the CI-NEB method. The potential energy surface of the structural evolution along the minimum energy pathways is shown in Figure 8. The first O migration, from the NO2 configuration to the NOSi configuration, occurs via the transition state TS1, shown in Figure 8, with a relatively small activation energy of 0.53 eV. We note here that this small barrier can be easily overcome by thermal energy at temperatures near room temperature. The energy barrier for the second O migration resulting in the change from NOSi to NSi2 is 0.79 eV, while the barrier from NOSi to NSi2-b is larger (1.06 eV, via the transition state TS2, shown in Figure 8). Once the second O has migrated to a Si backbond, the structural evolution to the most stable NSi2SiO2 configuration is relatively easy, with an energy barrier of 0.63 eV, as shown in the path from NSi2-b to NSi2-SiO2 (see Figure 8). The structural evolution from NSi2 to NSi2-SiO2 will 3015
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Table 1. Binding Energies (Eb) and Dipole Moments (μ) for Several Binding Configurations of Nitrobenzene on Si(001), at a Coverage of 1 MLa
a
The zero of Eb is defined for a gas phase nitrobenzene molecule at infinite separation from the Si(001) slab. A more negative Eb corresponds to more stable configurations. The positive (negative) sign indicates a dipole moment pointing away from (towards) the Si (001) surface.
pass through structure NSi2-b first and needs to overcome an energy barrier of 1.42 eV. In summary, the structural evolution from the initial NO2 configuration to the most stable NSi2-SiO2 structure can occur via two different paths, i.e., NO2 f NOSi f NSi2-bf NSi2-SiO2 and NO2 f NOSi f NSi2f NSi2-bf NSi2-SiO2, as shown in Figure 8. The most likely candidate for the rate limiting step along the first path is the migration of the second O, which occurs with an activation energy barrier of 1.06 eV, whereas the most likely candidate for the rate-determining step along the second path is the rearrangement of O atoms during the NSi2 f NSi2-b transformation, with an energy barrier of 1.42 eV. D. Dipole Moments. The dipole moment of nitrobenzene in the gas phase and of nitrobenzene adsorbed in various configurations on Si(001) was determined with DFT. In the gas phase, each nitrobenzene molecule has a substantial dipole moment of 4.5 D. The dipole moment of nitrobenzene adsorbed on Si(001), however, is far smaller than in the gas phase for all of the configurations we have studied. The atomic structures, binding energies, and dipole moments of different bonding configurations are summarized in Table 1. The significant reduction of the dipole moment of nitrobenzene upon adsorption on Si(001) can be explained by charge transfer from the Si dangling bonds to O atoms, in which an opposite interfacial dipole is developed. The interfacial dipole moment induced by the charge transfer points toward the surface and cancels most of the intrinsic dipole moment of the free nitrobenzene molecule which points away from the surface. The total dipole moment for each of the five structures ranges from 0.11 to 0.45 D. The lowest energy structure, NSi2-SiO2, has a dipole moment of 0.21 D.
4. DISCUSSION To elucidate the origin of the experimentally observed interruption in the periodic arrangement of nitrobenzene molecules, we compare the results of the DFT calculations with the STM measurements. The orientation of the phenyl ring is different in each of the bonding configurations identified by DFT. In structures NSi2-SiO2 and NSi2, N bonds to two Si atoms and the O atoms of the nitro group react with Si atoms to form Si-O bonds. In terms of thermodynamics, the most stable NSi2-SiO2 configuration would be expected to dominate in experiments. However, due to the kinetic limitations shown in the potential energy surface for structural evolution in Figure 8, the migration of O atoms of the nitro group in nitrobenzene may be incomplete
under our experimental conditions. As a result, the surface structure observed experimentally represents the coexistence of the NSi2-SiO2, NSi2 (and NSi2-b), and NOSi configurations, as well as perhaps the initial NO2 configuration. Different tilting orientations of phenyl rings in multiple bonding structures lead to variation in height of adsorbed nitrobenzene species. To test the hypothesis that the height variation in STM images arises from multiple configurations of nitrobenzene on the surface, we measured the distribution of the apparent heights of nitrobenzene molecules within an area of 100 100 Å2. The apparent heights of nitrobenzene species vary by up to 2.1 Å. Height histograms derived from STM images show that 40% of the nitrobenzene species have apparent heights between 1.1 and 1.7 Å, above the lowest molecule. This experimental height variation is consistent with the diversity of orientations of phenyl rings discovered in our calculations. The calculated difference in the heights of the phenyl rings of the initial NO2 nitrobenzene configuration and the most stable NSi2-SiO2 structure is 1.6 Å. The height of the topmost hydrogen atom of the phenyl ring in the NOSi, NSi2, NSi2-b, and NSi2-SiO2 configurations, relative to the respective position of that H in the NO2 configuration, is provided in Figure 8. Simulated STM images based on the structures obtained from DFT yield differences in height consistent with those shown in Figure 8. The short-range periodicity of molecular rows of nitrobenzene apparent in STM images thus likely arises from domains rich in the most stable NSi2-SiO2 configuration.
5. CONCLUSION We have presented a combined experimental and theoretical study that provides a molecular-level description of the structure, structural evolution, and electronic properties of nitrobenzene molecules on Si(001). The nitrobenzene monolayer on Si(001) lacks long-range order in STM images but shows signs of local periodic arrangement of molecules. Self-consistent, planewave DFT calculations show that the energy gained per molecule in the adsorption of nitrobenzene is nearly constant as a function of nitrobenzene coverage and that it is energetically favorable to adsorb nitrobenzene molecules on Si(001) with a coverage as high as one molecule per Si dimer. Nitrogen atoms are observed using X-ray photoelectron spectroscopy with a concentration approximately equal to the number of Si(001) dimers. Ab initio molecular dynamics (AIMD) simulations show that oxygen atoms migrate from an initial configuration in which the nitrobenzene 3016
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (M.M.);
[email protected] (P.E).
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’ ACKNOWLEDGMENT This work was supported by the University of Wisconsin Materials Research Science and Engineering Center through NSF Grant no. DMR-0520527 (G.P., M.M., R.H, R.R., and P.E.) and by the Petroleum Research Fund of the American Chemical Society (S.S. and P.E.). Computational resources were provided by the Department of Defense Supercomputing Centers (NAVY, ARSC, MHPCC, ERDC). ’ REFERENCES (1) Wehrli, S.; Poilblanc, D.; Rice, T. M. Eur. Phys. J. B 2001, 23, 345. (2) Taylor, D. M.; Bayes, G. F. Phys. Rev. E 1994, 49, 1439. (3) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda, T.; Mitani, T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, Y. Nat. Mater. 2004, 3, 317. (4) Campbell, I. H.; Rubin, S.; A., Z. T.; D., K. J.; Martin, R. L.; Smith, D. L. Phys. Rev. B 1996, 54, 14321. (5) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L. Appl. Phys. Lett. 1997, 71, 3528. (6) Park, B.; Paoprasert, P.; In, I.; Zwickey, J.; Colavita, P. E.; Hamers, R. J.; Gopalan, P.; Evans, P. G. Adv. Mater. 2007, 19, 4353. (7) Paoprasert, P.; Park, B.; Kim, H.; Colavita, P.; Hamers, R. J.; Evans, P. G.; Gopalan, P. Adv. Mater. 2008, 20, 4180. (8) Meyer zu Heringdorf, F. J.; Reuter, M. C.; Tromp, R. M. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 787. (9) Seo, S.; Grabow, L. C.; Mavrikakis, M.; Hamers, R. J.; Thompson, N. J.; Evans, P. G. Appl. Phys. Lett. 2008, 92, 153313. (10) Seo, S.; Evans, P. G. J. Appl. Phys. 2009, 106, 103521. (11) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33, 617. (12) Guisinger, N. P.; Greene, M. E.; Basu, R.; Baluch, A. S.; Hersam, M. C. Nano Lett. 2004, 4, 55. (13) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830. (14) Bocharov, S.; Mathauser, A. T.; Teplyakov, A. V. J. Phys. Chem. B 2003, 107, 7776. (15) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100. (16) Mendez De Leo, L. P.; Teplyakov, A. V. J. Phys. Chem. B 2006, 110, 6899. (17) Bocharov, S.; Teplyakov, A. V. Surf. Sci. 2004, 573, 403. (18) Meuris, M.; Mertens, P. W.; Opdebeeck, A.; Schmidt, H. F.; Depas, M.; Vereecke, G.; Heyns, M. M.; Philipossian, A. Solid State Technol. 1995, 38, 109. (19) Kim, H.; Colavita, P. E.; Paoprasert, P.; Gopalan, P.; Kuech, T. F.; Hamers, R. J. Surf. Sci. 2008, 602, 2382. (20) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11, 577. 3017
dx.doi.org/10.1021/jp1069434 |J. Phys. Chem. C 2011, 115, 3011–3017
