12599
2008, 112, 12599–12601 Published on Web 07/26/2008
Coverage Effect of Self-Assembled Polar Molecules on the Surface Energetics of Silicon Nira Gozlan and Hossam Haick* The Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion–Israel Institute of Technology, Haifa 32000, Israel ReceiVed: June 30, 2008
We show here that optimal control over the work function of Si can be achieved even with half-coverage of molecular dipoles, especially when the pinholes present on the surface are smaller than the depletion region of the semiconductor. Higher coverage than an optimal value contributes minor (or, no) further changes in the work function of the same semiconductor. The results imply that the requirements on the molecules used to control the electrical properties of Si are significantly and nearly completely relaxed. Therefore, the ability of the molecules to form pinhole-free coverage will not be important after a specific coverage. Controlling the electronic properties of Silicon (Si) is often a combination of science, technology and art, with the weight of each depending on how far the technology and understanding have advanced.1 To this end, a promising approach to control the electronic properties of Si is to use polar monolayers that introduce a net electrical dipole perpendicular to the surface/ interface2–4 and modify the work function and electron affinity at a surface and changes the band offset and band bending at an interface. This dipole effect is a general one and can be obtained with nonmolecular treatments as well.3 Nevertheless, the use of molecules, especially organic ones, allows a systematic tuning of the desired dipole moment by an appropriate choice of the functional group.2,4 Several experimental and theoretical works have dealt with the dipole influence on the electronic properties of Si (see refs 2, 4, and 5 and citations therein). However, almost no experimental works have been conducted with the aim to elucidate the influence of the layer’s coverage on it.5 Here, we systematically examine the coverage effect of adsorbed polar molecules on the surface energetics (work function) of Si that is coated with either negative or positive organic dipoles. We discuss and interpret the main findings by means of long-range electrostatic phenomena that are determined by the size, (dis-)order, and adsorption patterns within the monolayer. Monolayers of butyltrichlorosilane (Cl3Si(CH2)3-CH3; BTS) and 3-bromopropyl)trichlorosilane (Cl3Si(CH2)3-Br; BPTS) with varied surface coverage (∼10-98%) were formed by self-assembly on (2-3 nm) native oxide film (SiOx) on n-Si (1015 cm-3) substrates6 in a dry nitrogen glovebox. These molecules have a common (C3 alkyl) backbone and (silane) binding groups, but differ with their exposed functional groups (CH3 versus Br), to modify the electron donating/ withdrawing power and polarity. Desnity functional theory (DFT) calculations using the GO3W package (B3LYP method and 6-311G basis set), revealed dipole moment values of -2.80 D for BTS and +2.05 D for BPTS free molecules. Note: the dipole is defined as positive if its positive pole is * To whom correspondence should be addressed. E-mail: hhossam@ technion.ac.il.
10.1021/jp805723u CCC: $40.75
Figure 1. AFM images showing several phases formed in the selfassembly processes of BTS on (native) SiOx/Si at different coverage: (a) ∼24%; (b) ∼50%; (c) ∼87%; and (d) ∼97%. The AFM cross section profiles of the molecular “domains” showed comparable thickness values to those obtained by ellipsometery. The scale in all images is 100 nm × 100 nm.
closest to the substrate. Surface analysis for all samples were carried out as reported earlier.6 Figure 1 shows representative atomic force microscopy (AFM) images of BTS monolayers at different coverage on the SiOx/Si surfaces. At ∼24% coverage (Figure 1a) the BTS arranged in nucleated islands having a characteristic diameter of 5-20 nm and separated from the other by 40-50 nm. The dimensions and height of the islands increased with increasing the surface coverage, indicating that BTS molecules changed their tilting continuously. That is so because molecules lying flat on the surface gradually grow two-dimensionally to the standing-up phase, as the surface coverage is increased.7 At ∼50% coverage, the characteristic diameter of the molecular 2008 American Chemical Society
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Letters
Figure 2. Work function (Φ) of SiOx/Si substrates as a function of BTS and BPTS coverage. At zero coverage, the work function stands on 4.61 ( 0.05 eV. The presented data are the average of at least three different samples.
domains increased to 20-25 nm while showing better uniformity than those appeared at a lower coverage. Furthermore, at ∼50% coverage, pinholes (i.e., molecule-free domains) having characteristics diameter of 5-15 nm were evolved. At coverage values higher than ∼50%, the interisland regions (pinholes) shrank and eventually a continuous and ordered SAM was formed (Figures 1c,d). The average coverage values obtained by AFM were comparable to those obtained from our X-ray photoelectron spectroscopy (XPS) experiments on the same samples. For BPTS on SiOx/Si surfaces, the AFM and XPS observations were similar to those of the BTS/SiOx/Si case. The change of the work function (Φ) with respect to Au reference (hereby, ∆Φ) was determined for the same samples characterized by AFM. For molecules on a SiOx/Si surface, ∆Φ can be expressed as follows8
∆Φ )
N · µ · cos θ εεo
(1)
Here, µ is the molecule’s dipole moment, N is the density of adsorbed molecules; θ is the average tilt of molecules relative to the surface normal; ε is the effective dielectric constant of the molecular film (including any depolarization effects5); εo is the permittivity of free space. The work function varied with the free molecules’ dipole moments, in agreement to previous reports.4,9 The molecularly modified SiOx/Si surfaces showed clear but opposite effects of the molecule substituent on the Φ characteristics (Figure 2). At a specific coverage, negative molecular dipoles (BTS; -2.80 D) decreased the Φ of SiOx/Si and positive molecular dipoles (BPTS; +2.05 D) increased it (Figure 2). From Figure 2, one can also observe that upon deposition of BTS the Φ decreased rapidly by 0.24 eV reaching 4.37 eV at ∼50% coverage. Also, one can observe that upon deposition of BPTS, the Φ increased rapidly by 0.29 eV, reaching 4.90 eV at ∼42% coverage. The Φ change in these regions is likely due to a partial electron transfer from BPTS, the electron donor, to the SiOx/Si surface atoms upon chemisorptions, and vice versa for the BTS molecules. This charge redistribution creates a chemical dipole that contributes, together with the modification of the electronic density at the SiOx/Si surface, to the overall interface dipole when a monolayer is chemisorbed on SiOx/Si. From the critical coverage on, until reaching almost full coverage, the work functions of the modified SiOx/Si did not depend strongly upon the coverage of the molecules. Indeed, structures having coverage above the critical value showed an average work function of 4.34 eV for BTS/SiOx/Si and 4.92 eV for BPTS/SiOx/Si. This is slightly surprising since one would expect a higher coverage to result in a larger work function shift because of a higher density of
molecular dipoles (see eq 1). These results can be understood, however, if one consider the dioples as a source for electrostatic effects that extend into the adjacent molecule-free domains,5,9 as will be discussed in the following paragraph. Electrostatics teaches us that dipolar domains affect the semiconductor below the areas where there are no molecules (i.e., the pinholes).5,10 When the lateral dimensions of a pinhole are greater than the depletion width, the work function of the pinholes (Φpinhole) is not influenced by the molecular layer and the “effective” work function is simply the arithmetic average of Φpinhole and Φdomain. We estimate that for Si with Nd ∼ 1015 cm-3, the work function of pinholes with diameters larger than their depletion region, viz. >30 nm, is not affected by the molecular dipole layer nearby (as a representative example cf. the case of BTS/SiOx/Si at ∼24% coverage). However, if the lateral inhomogeneity in the work function occurs on a length scale smaller than the semiconductor’s depletion width (as is the case in >50 and >42% coverage of BTS and BPTS on SiOx/ Si surfaces, respectively),11 the depletion regions will extend laterally in the semiconductor sufficiently so as to dampen the potential variations in the space charge region. The most pronounced effect of such extended space charge regions will be the apparent increase of the effective work function for regions with nominally low work function, due to the presence of high work function regions close by. This effect, loosely termed “pinch-off”, thus offers a mechanism by which the molecular layer may be used to control and tune the transport properties of the pinholes.9,12 If the work function in the monolayer-semiconductor domains is less than that in the pinholes, that is, Φdomain < Φpinhole, pinch-off can occur under the domains instead of under the pinholes, that is, the problem is qualitatively a symmetric one.10 A full analysis of these remarkable effects, supported by further experiential results, will be published elsewhere. In summary, poorly organized, incomplete, molecular monolayers can be considered for controlling electronic devices because a layer of dipoles affects the interface beyond the lateral dimensions of the molecular domain, that is, into the semiconductor, reaching also below the adjacent pinhole areas, located between adjacent molecular pinholes. This molecular effect depends to a large extent on the molecular coverage on the surface and the concentration and size of both large pinholes and small pinholes. Optimal control over the work function of SiOx/Si can be achieved even with half-coverage of molecular dipoles, especially when the pinholes exist on the surface are smaller than the depletion region of the semiconductor. Putting these results in wider prospective, the requirements on the molecule on SiOx/Si surfaces are significantly and nearly completely relaxed. Therefore, the ability of the molecules to form pinhole-free coverage will not be important. In addition, formation of laterally inhomogeneous molecular layers is in the direction of and may lay the foundation for the implementation of multiple functions on hard or flexible substrates with lateral resolution down to tens of nanometers. Acknowledgment. We acknowledge the Marie Curie Excellence Grant of the FP6, the U.S.-Israel Binational Science Foundation, and the Russell Berrie Nanotechnology Institute for financial support. H.H. holds the Horev Chair for the Leaders in Science and Technology. References and Notes (1) (a) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Anal. Chim. Acta 2006, 568, 84. (b) Puniredd, S. R.; Assad, O.; Haick, H. J. A. Chem. Soc. 2008, 130, 9184.
Letters (2) Anagaw, A. Y.; Wolkow, R. A.; DiLabio, G. A. J. Phys. Chem. C. 2008, 112, 3780. (3) Franciosi, A.; van deWalle, C. G. Surf. Sci. Rep. 1996, 25, 1. (4) Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121. (5) Natan, A.; Kronik, L.; Haick, H.; Tung, R. T. AdV. Mater. 2007, 19, 4103. (6) Gershewitz, O.; Grinstein, M.; Sukenik, C. N.; Regev, K.; Ghabboun, J.; Cahen, D. J. Phys. Chem. B 2004, 108, 664. (7) Schwartz, D. K. Annu. ReV. Phys. Chem. 2001, 52, 107. (8) Moench, W. Semiconductor surfaces and interfaces, 3rd edition; Springer-Verlag: Berlin, Germany, 2001.
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12601 (9) Haick, H.; Ambrico, M.; Ligonzo, T.; Tung, R. T.; Cahen, D. J. Am. Chem. Soc. 2006, 128, 6854. (10) Haick, H.; Pelz, J. P.; Ligonzo, T.; Ambrico, M.; Cahen, D.; Wei, C.; Marginean, C.; Tivarus, C.; Tung, R. T. Phys. Status Solidi A 2006, 203, 3438. (11) The higher coverage required for BTS to reach the saturation level of work function, as compared with BPTS, could be attributed to a higher depolarization effect in the BTS films, due to a higher dipole moment values. This depolarizing effect acts to decrease (or tend to cancel) the dipoles and therefore the work function shift. (12) Tung, R. T. Phys. ReV. B 1992, 45, 13509.
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