UV Photooxidation of a Homologous Series of n ... - ACS Publications

Jan 3, 2008 - canethiol, and hexadecanethiol, decompose. In agreement with previous studies,39 for short-chain alkanethiols (n e 15) adsorbed...
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J. Phys. Chem. C 2008, 112, 797-805

797

UV Photooxidation of a Homologous Series of n-Alkanethiolate Monolayers on GaAs(001): A Static SIMS Investigation Chuanzhen Zhou and Amy V. Walker* Department of Chemistry and Center for Materials InnoVation, Washington UniVersity in St. Louis, Campus Box 1134, One Brooking DriVe, St. Louis, Missouri 63130 ReceiVed: July 25, 2007; In Final Form: October 25, 2007

We have investigated the UV photooxidation of a homologous series of methyl-terminated self-assembled alkanethiolate monolayers adsorbed on GaAs(001) using time-of-flight secondary ion mass spectrometry. We observe that for undecanethiol (UDT) and tetradecanethiol (TDT), the reaction kinetics are well-described by pseudo-first-order rate constants. For hexadecanethiol (HDT), octadecanethiol (ODT), and eicosanethiol (ECT), there is a change in the apparent kinetics from first-order to second-order. In agreement with previous studies of the photooxidation of self-assembled monolayers (SAMs) adsorbed on metals, for short-chain alkanethiols (n e 15) adsorbed on GaAs(001), the rate determining step is the penetration of oxygen to the substrate/S interface. However, for longer-chain alkanethiols, the photooxidation rate is not solely determined by the penetration of the active oxygen species to the interface. The kinetic analyses indicate that the reaction proceeds via the simultaneous photooxidation of the alkanethiols to alkylsulfonates and alkylsulfates as well as the decomposition of the alkanethiols. In the case of UDT, TDT, and HDT, once formed, the alkylsulfonates can also oxidize to form alkylsulfates or decompose. Also, unlike the case of SAMs adsorbed on metals, the GaAs substrate photooxidizes. The rates of photooxidation of SAMs adsorbed on GaAs(001) are higher than those for SAMs adsorbed on Au. This can be related to differences in the structures of methyl-terminated SAMs adsorbed on GaAs(001) and Au.

1. Introduction Self-assembled monolayers (SAMs) adsorbed on semiconductors are of interest for both fundamental studies and technological applications.1-14 SAMs have been demonstrated to passivate GaAs, preventing the oxidation of the surface by water.3 They have also been employed as resist masks for lithography on GaAs and Si.11-13 Further, SAMs can be used to alter the electrical properties of semiconductors4,14 by, for example, changing the interface state density.14 Gallium arsenide is the most widely used III-V semiconductor.15 There have been a number of studies of SAM formation on GaAs using alkanethiols1,4,5,14,16-22 and aromatic thiols.23-25 The formation of SAMs on GaAs is very sensitive to the experimental conditions employed,20 and therefore, there is much variation between reported monolayer structures. For example, for octadecanethiol adsorbed on GaAs(001), chain tilt angles varying from 5716 to 1420 degrees have been measured. Recently, McGuiness and co-workers have reported that well-ordered SAMs can be reproducibly achieved on GaAs(001) from an ethanolic alkanethiol solution under carefully controlled experimental conditions.20,22 The nature of the bonding between the GaAs substrate and the alkanethiol remains controversial, with reports that the thiol binds via Ga-S,26 As-S,17,27,28 and both Ga-S and As-S bonding.22,29 Methods that have been employed for producing patterned SAMs include microcontact printing,30-32 dip pen nanolithography,32 nanoimprinting,31,32 and UV photopatterning.33-39 UV photopatterning is a simple, convenient method for producing * To whom correspondence should be addressed. E-mail: walker@ wustl.edu. Ph: 314 935 8496. Fax: 314 935 4481.

chemically well-defined, contamination-free, micron-scale SAM patterns with low defect densities on metals. In this method, UV light is shone through a mask onto an alkanethiolate SAM surface. In the areas exposed to UV light, the SAM is photooxidized and can be replaced with a second SAM, creating a micron-scale patterned surface. This method can be extended to the nanometer scale using scanning near-field photolithography (SNP).40-42 Recent experiments have demonstrated that UV photopatterning is an effective method for producing patterned SAMs adsorbed on GaAs(001).39 Although there have been many studies of the UV photopatterning of SAMs adsorbed on metals, 33-38 the mechanism of SAM photooxidation remains controversial. Several mechanisms have been proposed, including ozonolysis,43,44 formation of singlet oxygen species,45 and hot electron attachment.36,38,46 There have been few studies of the reaction pathways involved in the photooxidation of SAMs adsorbed on GaAs.39 Unlike metals, GaAs is easily photooxidized upon exposure to UV light and oxygen,47 which are the experimental conditions used to photooxidize alkanethiolate SAMs. It is likely that the mechanism of UV photooxidation of SAMs adsorbed on GaAs is therefore different than that observed for SAMs adsorbed on metals. In this paper, we systematically investigate the UV photooxidation of a homologous series of methyl-terminated alkanethiolate monolayers adsorbed on GaAs(001) using timeof-flight secondary ion mass spectrometry (TOF SIMS). The kinetic analyses suggest that the reactions proceed via the simultaneous photooxidation of the alkanethiols to alkylsulfonates and alkylsulfates as well as the decomposition of the alkanethiols. Once formed, the alkylsulfonates can also oxidize to form alkylsulfates or, in the case of undecanethiol, tetrade-

10.1021/jp075863u CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008

798 J. Phys. Chem. C, Vol. 112, No. 3, 2008 canethiol, and hexadecanethiol, decompose. In agreement with previous studies,39 for short-chain alkanethiols (n e 15) adsorbed on GaAs(001), we find that the rate-determining step is the penetration of oxygen to the GaAs/S interface. Unlike the photooxidation of SAMs on metals, the reaction mechanism changes as the number of carbons in the methylene chain increases. For alkanethiols with more than 16 carbons, the reaction pathway is not solely determined by the diffusion of the active oxygen species through the monolayer. Other factors that may influence the reaction mechanism include the restructuring of the underlying GaAs(001) substrate. Finally, we show that the rates of photooxidation of SAMs adsorbed on GaAs(001) are higher than those of SAMs on Au, which can be related to differences in the monolayer structure of alkanethiols adsorbed on GaAs(001) and Au. 2. Experimental Section 2.1. Materials. Undecanethiol (UDT) (99%) and hexadecanethiol (HDT) (99%) were purchased from Asemblon (Redmond, WA). Tetradecanethiol (TDT) (98%) and octadecanethiol (ODT) (98%) were purchased from Fluka (Milwaukee, WI) and Aldrich (St. Louis, MO), respectively. Eicosanethiol (ECT) was synthesized and supplied by D. L. Allara, Pennsylvania State University.48 All alkanethiols were used without any further purification. Single-side polished n-type GaAs(001) wafers (Si dopant, (0.8-4) × 1018/cm3) (American Xtal Technologies, Fremont, CA) were used for all studies. Native oxide Si wafers were obtained from Addison Technologies, Inc. (Pottstown, PA) and were cleaned using piranha etch before use. Chromium and gold were purchased from Goodfellow (Oakdale, PA) and Alfa Aesar (Ward Hill, MA), respectively, and were of greater than 99.99% purity. The 30% ammonium hydroxide in water was purchased from JT Baker (CMOS grade) (Phillipsburg, NJ), and anhydrous ethanol (ACS/USP grade) was obtained from Aaper Alcohol (Shelbyville, KY). 2.2. SAM Preparation. Methyl-terminated SAMs with different alkyl chain lengths were prepared on GaAs using the procedure reported by McGuiness et al.20 Briefly, GaAs substrates were immersed in ammonium hydroxide for 5 min to remove the native oxide layer, rinsed with degassed ethanol, and dried using nitrogen gas. The GaAs substrates were then immersed in degassed 3 mM ethanolic solutions of the corresponding alkanethiol (UDT, TDT, HDT, ODT, and ECT) and ∼10 mM ammonium hydroxide and transferred into a nitrogenpurged glovebox. The substrates were immersed in the solution for approximately 20 h at ambient temperature (21 ( 2 °C). After removal from solution, the samples were rinsed with copious amounts of degassed ethanol and dried using nitrogen gas. Single-wavelength ellipsometry measurements were taken to obtain the film thickness. In agreement with the measurement of McGuiness et al.,20 a value of 22 ( 1 Å for ODT was obtained. The measured ellipsometric thicknesses for UDT, TDT, HDT, and ECT were 12 ( 1, 17 ( 1, 19.5 ( 1.0, and 26.8 ( 1.0 Å, respectively. The preparation and characterization of methyl-terminated SAMs on Au have been described in detail previously.49-52 Briefly, first Cr (∼50 Å) and then Au (∼1000 Å) were thermally deposited onto clean native oxide Si wafers. Self-assembly of well-ordered monolayers was achieved by immersing the resulting Au substrate into a 1 mM degassed ethanolic solution containing the corresponding alkanethiol (UDT, TDT, HDT, ODT, and ECT) for 24 h at ambient temperature (21 ( 2 °C). To ensure that the prepared SAMs were well-ordered and free from significant chemical contamination prior to UV photooxi-

Zhou and Walker dation for each batch, a sample (∼1 × 1 cm2) was taken and characterized using single-wavelength ellipsometry and timeof-flight secondary ion mass spectrometry (TOF SIMS). 2.3. Photooxidation of SAMs on GaAs. The samples were placed approximately 50 mm away from a 500 W Hg arc lamp equipped with a dichroic mirror (to remove IR light) and a narrow-band-pass filter (280-400 nm) (Thermal Oriel, Spectra Physics Inc., Stratford, CT). SAMs adsorbed on GaAs(001) were exposed to UV light for 5, 10, 20, 30, 40, 60, and 90 min. SAMs adsorbed on Au were exposed to UV light for 5, 10, 20, 30, 40, 60, 90 120, 180, 240, and 300 min. After photoreaction, the SAM samples were immediately transferred to the TOF SIMS instrument for analysis. For each data point, three different samples were prepared, and at least three spots on each sample surface were analyzed. Each datum presented is therefore an average over at least nine measurements, and the uncertainty given is the standard deviation. 2.4. Time-of-Flight Secondary Ion Mass Spectrometry. TOF SIMS analyses were conducted using a TOF SIMS IV (ION TOF Inc., Chestnut Ridge, NY) instrument equipped with a Binm+(n ) 1-7, m ) 1, 2) liquid metal ion gun. The instrument consists of an airlock, a preparation chamber, and an analysis chamber, separated by gate valves. The preparation and analysis chambers were maintained at less than 5 × 10-9 mbar to avoid sample contamination. The primary Bi+ ions had a kinetic energy of 25 keV and were contained within a ∼100 nm diameter probe beam. For high-mass-resolution spectra acquisition, the Bi+ ion beam was rastered across a (100 × 100) µm2 area. The total accumulated primary ion dose was less than 1 × 1010 ions/cm2, which is within the static SIMS regime.53 The secondary ions were extracted into a time-of-flight mass spectrometer and reaccelerated to 10 keV before reaching the detector. Peak intensities were reproducible to within (8% from sample to sample and (6% from scan to scan. 3. Results and Discussion We note that the positive and negative ion TOF SIMS spectra of UDT and ODT adsorbed on GaAs(001) have been discussed previously in refs 22 and 39. They are included here for completeness. 3.1. TOF SIMS Characterization of Methyl-Terminated SAMs Adsorbed on GaAs(001). For all SAMs studied, the negative ion intensities of SOx- (x ) 1-4) provide useful information that the SAMs were prepared without substantial incorporation of impurities and that the molecular layers were not initially oxidized. In the TOF SIMS spectra, we observe ions of the form GaxOy( and AsxOy(, suggesting that the underlying GaAs substrate is partially oxidized (see Supporting Information). Further, the intensities of these ions decrease with increasing chain length, in agreement with previous studies. 22,48 Using X-ray photoelectron spectroscopy (XPS), McGuiness et al. observed for HDT and dodecanethiol (DDT) SAMs that ∼1 and ∼2% of the total As 3d signals were due to AsOy species.48 The positive ion mass spectra of the bare methyl-terminated monolayers show a number of high-mass molecular cluster ions of the form 69GaxMy+, 71GaxMy+, and AsxMy+, where M ) -S(CH2)nCH3 (UDT, n ) 10; TDT, n ) 13; HDT, n ) 15; ODT, n ) 17; ECT, n ) 19) (data not shown). Common fragment ions observed include (CH2)x+, (CH2)xCH+, CH3(CH2)+, S(CH2)x+, and GaxAsySzH+. In the negative ion mass spectra, the intact molecular ions, M-, and cluster ions such as 69GaSM2-, 71GaSM2-, and AsSM2are observed. Other common characteristic fragment ions are CH(CH2)xS-, CH3(CH2)xS-, and GaxAsySz-.

Photooxidation of SAMs Adsorbed on GaAs(001)

Figure 1. TOF SIMS negative ion mass spectra of TDT adsorbed on GaAs(001) upon exposure to UV light for (a) 0, (b) 5, (c) 20, and (d) 40 min.

In agreement with previous studies,22,39 the mass spectra indicate that the alkanethiolates are bound to the GaAs surface via both Ga-S and As-S bonds. For all SAMs studied, we observe ions of the form GaS(, AsS(, GaxAsySzH+, GaxAsySz-, GaM+, and AsM+. 3.2. Photooxidation of Methyl-Terminated SAMs Adsorbed on GaAs(001). Negative ion TOF SIMS mass spectra were obtained for UDT, TDT, HDT, ODT, and ECT SAMs exposed to UV light in the presence of oxygen for certain periods of time. As an example of the data obtained, Figure 1 shows negative ion mass spectra of TDT adsorbed on GaAs(001) after exposures of 0, 5, 20, and 40 min. For all SAMs studied, after 5 min of UV exposure, we observe a new ion at m/z ) 235, 277, 305, 333, and 361 for UDT, TDT, HDT, ODT, and ECT, respectively. These are identified as oxidized SAM molecules, MSO3-, of chemical composition CH3(CH2)nSO3with n ) 10 (UDT), 13 (TDT), 15 (HDT), 17 (ODT), and 19 (ECT). At the same time, the intensities of the corresponding molecular and cluster ions decrease. Other ions indicative of photooxidation also appear, including SO2- (m/z ) 64) and SO3- (m/z ) 80). Ions are observed that indicate that the GaAs substrate has also oxidized, including 69GaOx-, 71GaOx-, and AsOx- (x ) 1, 2). Further, after 5 min of UV exposure, the AsS- (m/z ) 106) ion intensity disappears for UDT, TDT, HDT, and ODT. In contrast, for ECT, the disappearance of the AsSion (m/z ) 106) takes a longer time. For all monolayers, the

J. Phys. Chem. C, Vol. 112, No. 3, 2008 799 intensities of GaS- (m/z ) 101, 103) also decrease. We also note that the intensities of oxygen-containing ions such as OH(m/z ) 17) have generally increased after 5 min of UV exposure. With longer UV exposure times (Figure 1 and Supporting Information), the ion intensities of MSO3-, SO2-, and SO3continue to increase, while the intensity of the molecular, M-, and cluster ions continues to decrease. New ions appear, including SO4- (m/z ) 96) and MSO4- (M ) -S(CH2)nCH3; UDT, n ) 10, m/z ) 251; TDT, n ) 13, m/z ) 293; HDT, n ) 15, m/z ) 321; ODT, n ) 17, m/z )349; ECT, n ) 19, m/z ) 377), which suggests that the SAM continues to oxidize. We also observe a series of fragment ions of the general formula CH3(CH2)n-2CHdCHSO3- (UDT, n ) 3-9; TDT, n ) 3-12; HDT, n ) 3-14; ODT, n ) 3-16; ECT, n ) 3-18) (Figure 1c), indicating that the SAMs are decomposing. Similar fragment ions are found in the mass spectra of photooxidized SAMs adsorbed on Au,54 which suggests that a related reaction mechanism is operative. As the UV exposure time is increased, the ion intensities of MSO3- and MSO4- increase, while the ion intensity of Mdecreases (Figure 1). The intensity of the molecular ion, M-, decreases to zero after 20, 40, 60, 90, and 120 min of UV exposure for UDT, TDT, HDT, ODT, and ECT, respectively, indicating that the photooxidation process is complete. 3.3. Kinetics of the Photooxidation of Methyl-Terminated SAMs Adsorbed on GaAs(001). The initial photooxidation of methyl-terminated SAMs adsorbed on GaAs (001) is similar to that observed for SAMs adsorbed on metals;36-38,46,54,55 upon exposure to UV light, the thiol group oxidizes to form a sulfonate, -SO3. However, unlike SAMs adsorbed on metals, SAMs adsorbed on GaAs(001) continue to oxidize, forming sulfates, -SO4. The underlying GaAs substrate also oxidizes, forming a nonstoichiometric oxide, which is a mixture of Ga2O3, As2O3, and As2O5.47 To further investigate the reactions involved in the photooxidation of methyl-terminated SAMs adsorbed on GaAs(001), a quantitative analysis of the SIMS data was performed using the relative ion intensity method first described by Vickerman and co-workers.56,57 We note that the quantitative analysis of SIMS ion intensities is controversial. However, this method has been successfully employed to study the photooxidation of SAMs adsorbed on metals.35,36,38,45,54,55 Importantly, reaction rates obtained using SIMS54 and XPS45 for photooxidation of methyl-terminated SAMs adsorbed on Au were found to be the same. In the relative ion intensity method, for a reaction

A+BfC+D where the concentration of B is in excess, it is assumed that the ion intensities of the reactant A and the product of interest, C, are proportional to the species concentration, that is, the ion formation cross section is the same for A and C. The extent of reaction is given by

χ(t) )

[C] [A] + [C]

where [A] and [C] are the ion intensities of A and C, respectively, and χ(t) ) 1 indicates that the reaction is complete. Simple kinetic analyses of χ(t) can then be performed to determine whether the data are consistent with first-order or second-order kinetics. The fraction of the reactant A still present at time t is given by

χ′(t) ) 1 - χ(t)

800 J. Phys. Chem. C, Vol. 112, No. 3, 2008

Zhou and Walker TABLE 1: Calculated Rate Constants for the Photooxidation of UDT, TDT, HDT, ODT, and ECT to Alkylsulfonates

SAM UDT TDT HDT ODT ECT Figure 2. Extent of the photooxidation χ1(t) of UDT, TDT, HDT, ODT, and ECT adsorbed on GaAs(001) as a function of UV exposure time. The lines are the best kinetic fits to the experimental data, first-order for UDT, TDT, and HDT and second-order for ODT and ECT.

first-order rate constant (min-1) for M f MSO3a

second-order rate constant (min-1) for M f MSO3a

(2.0 ( 0.3) × 10-1 (1.0 ( 0.1) × 10-1 (8.3 ( 0.8) × 10-2 (1.3 ( 0.2) × 10-1; (1.6 ( 0.1) × 10-2 (3.56 ( 0.01) × 10-2

(7.8 ( 0.1) × 10-1 (1.8 ( 0.1) × 10-1 (1.1 ( 0.2) × 10-1

a Uncertainties are obtained from linear regression analysis at 95% confidence.

If the kinetics are pseudo-first-order, the following integrated rate law will apply

ln χ′(t) ) ln χ′(0) + kt If the kinetics are pseudo-second-order, the corresponding integrated rate law will apply

1 1 ) + kt χ′(t) χ′(0) where k is the effective rate constant. Since χ(t) is unitless, both rate constants have dimension of (time)-1. While this approach is not a comprehensive kinetic analysis, it does provide a robust definition of an effective rate constant, which may then be used to compare reactions of different SAMs adsorbed on GaAs(001) and other substrates.38,39,45,54 Kinetics of the Photooxidation of Methyl-Terminated SAMs. Initially, the methyl-terminated SAM photooxidizes to form a sulfonate, -SO3. The underlying GaAs substrate also oxidizes, forming a nonstoichiometric oxide which is a mixture of Ga2O3, As2O3, and As2O5.47 Thus, the photooxidation reaction occurs according to the (unbalanced) reaction equation39

CH3(CH2)nSads + GaAs + hν

O2 98 CH3(CH2)nSO3- + [GaxAsyOz]+ (1) where the subscript “ads” indicates an adsorbed species. As explained above, the extent of photooxidation in reaction 1, χ1(t), at time t is obtained from the ratio of the sulfonate ion intensity, [MSO3-], to that of the unoxidized molecular ion intensity, [M-], by χ1(t) ) [MSO3-]/([M-] + [MSO3-]). Figure 2 displays the extent of photooxidation, χ1(t), versus UV exposure time for all of the methyl-terminated SAMs studied. As in previous studies of SAMs adsorbed on metals,45,54,55 the completion time (χ1(t) ) 1) for the photoreaction is dependent on the methylene chain length, with shorter alkanethiolates oxidizing faster than longer-chain SAMs. Data for UDT and TDT are well described by pseudo-firstorder kinetics (Table 1 and Figure 3a). However, the HDT, ODT, and ECT data can be fit to both first-order and secondorder kinetics (Table 1; Figure 3b and c). Rate constants were obtained by linear regression of ln χ1′(t) and 1/χ1′(t) versus t for first-order and second-order reactions, respectively. The ODT data could not be fit with a single first-order process. In order to obtain a pseudo-first-order rate constant for comparison with other SAMs, we fit the data to two sequential first-order rate constants, as in ref 39.

Figure 3. Variation of ln χ1′(t) with UV exposure time for (a) UDT and TDT and (b) HDT, ODT, and ECT adsorbed on GaAs(001). (c) Variation of 1/χ1′(t) with UV exposure time for HDT, ODT, and ECT adsorbed on GaAs(001).

Unlike the photoreaction of alkanethiolate SAMs on metals,36-38,46,54,55 we observe that methyl-terminated SAMs on GaAs continue to oxidize, forming sulfates, MSO4 (Figure 1). Figure 4 displays the negative ion mass spectra for TDT, HDT, ODT, and ECT at the same extent of photooxidation, χ1(t) ) 0.72. The data for UDT are not shown because the photooxidation reaction rate is very fast; after 5 min of UV exposure, the photooxidation of UDT is already ∼90% complete (χ1(t) ∼ 0.9) (Figure 2). For ODT and ECT, the intensity of the MSO4ion is much larger than that for TDT or HDT, suggesting that the photooxidation reaction pathway for ODT and ECT is

Photooxidation of SAMs Adsorbed on GaAs(001)

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Figure 5. (a) Variation of ln χ2′(t) with UV exposure time for UDT, TDT, HDT, ODT, and ECT adsorbed on GaAs(001). (b) Variation of 1/χ3′(t) with UV exposure time for ODT and ECT adsorbed on GaAs(001).

TABLE 3: Calculated Rate Constants for the Photooxidation of UDT, TDT, and HDT from Alkylsulfonate, MSO3, to Alkylsulfate, MSO4

Figure 4. Negative ion TOF SIMS spectra for TDT, HDT, ODT, and ECT adsorbed on GaAs(001) at an extent of photooxidation of χ1(t) ) 0.72.

TABLE 2: Calculated Rate Constants for the Photooxidation of UDT, TDT, HDT, ODT, and ECT to Alkylsulfates

SAM

first-order rate constant (min-1) for M f MSO4a

second-order rate constant (min-1) for M f MSO3a

UDT TDT HDT ODT ECT

(6.0 ( 0.1) × 10-2 (6.6 ( 0.1) × 10-2 (5.6 ( 0.1) × 10-2 (1.9 ( 0.2) × 10-2 (6.2 ( 1.1) × 10-3

(3.4 ( 0.1) × 10-2 (7.5 ( 0.1) × 10-3

a

Uncertainties are obtained from linear regression analysis at 95% confidence.

different than that for the shorter-chain alkanethiols. There are at least two possible reaction pathways for the formation of alkylsulfate. The first is the simultaneous photooxidation of the methyl-terminated SAMs to alkylsulfonates and alkylsulfates. The (unbalanced) reaction equation is

CH3(CH2)nSads + hν

O2 + e- 98 CH3(CH2)nSO3- + CH3(CH2)nSO4- (2) In this case, the extent of reaction is given by χ2(t) ) [MSO4-]/ ([M-] + [MSO4-]). The χ2(t) data for all monolayers studied can be fit to a first-order decay (Table 2; Figure 5a). However, a better fit is obtained for ODT and ECT if two first-order reaction rate constants are used, as in the case of χ2(t). We note

SAM

first-order rate constant (min-1) for MSO3 f MSO4a

second-order rate constant (min-1) for MSO3 f MSO4a

UDT TDT HDT

(1.4 ( 0.2) × 10-3 (6.1 ( 0.8) × 10-3 (5.1 ( 0.3) × 10-3

(1.4 ( 0.2) × 10-3 (7.0 ( 0.8) × 10-3 (5.9 ( 0.4) × 10-3

a Uncertainties are obtained from linear regression analysis at 95% confidence.

that the χ2(t) data for ODT and ECT are also consistent with a second-order reaction (Table 2; Figure 5b). A second possible pathway for the formation of the alkylsulfate is the oxidation of the alkylsulfonate hν CH3(CH2)nSO3,ads + 21O2 + e- 98CH3(CH2)nSO4- (3)

The extent of reaction, χ3(t), is given by χ3(t) ) [MSO4-]/ ([MSO3-] + [MSO4-]). Data for UDT, TDT, and HDT can be fit to both first-order and second-order decays (Table 3; Figure 6), suggesting that the alkylsulfate, MSO4, may also form directly from the alkylsulfonate, MSO3. For ODT and ECT, χ3(t) data are inconsistent with either a single first-order or second-order reaction rate. Taken together the χ1(t), χ2(t), and χ3(t), the data indicate that the primary reaction pathway for the formation of the alkylsulfate is the photooxidation of the molecule directly to the alkylsulfate. For UDT, TDT, and HDT, the rate constants for the formation of MSO4 from the SAM molecule are at least 10 times larger than the rate constants for the formation of MSO4 from MSO3 (Tables 2 and 3). For ODT and ECT, the kinetic analysis indicates that the reaction pathway involved in the formation of the alkylsulfate from the alkylsulfonate does not have a simple dependence on the concentration of the alkylsulfonate. The data also suggest that for UDT, TDT, and HDT,

802 J. Phys. Chem. C, Vol. 112, No. 3, 2008

Zhou and Walker

Figure 6. (a) Variation of ln χ3′(t) with UV exposure time for UDT, TDT, and HDT adsorbed on GaAs(001). (b) Variation of 1/χ3′(t) with UV exposure time for UDT, TDT, and HDT adsorbed on GaAs(001).

TABLE 4: Calculated Rate Constants for the Decomposition of UDT, TDT, and HDT to Form C6H11SO3 from the Alkylsulfonate, MSO3

SAM

first-order rate constant (min-1) for MSO3 f C6H11SO3a

second-order rate constant (min-1) for MSO3 f C6H11SO3a

UDT TDT HDT

(5.6 ( 1.1) × 10-3 (4.5 ( 0.4) × 10-3 (2.9 ( 0.2) × 10-2

(6.0 ( 1.1) × 10-3 (5.0 ( 0.4) × 10-3 (3.2 ( 0.2) × 10-3

a

Uncertainties are obtained from linear regression analysis at 95% confidence.

there is a secondary pathway for the formation of alkylsulfates, the photooxidation of the alkylsulfonate. In the negative ion mass spectra (Figure 1), we observe ions of the general formula CH3(CH2)n-2CHdCHSO3-, which suggest that the methyl-terminated SAMs are decomposing on the surface as well as photooxidizing.54 Such ions can arise either from (A) decomposition of the already oxidized alkanethiolate or (B) from photooxidation of the already decomposed alkanethiolate. We determined the extent of these reactions using one characteristic ion, C6H11SO3- (CH3(CH2)3CHdCHSO3-). Using other fragment ions of this general formula gave similar results. For case A, the extent of reaction is given by χ4(t) ) [C6H11SO3-]/([MSO3-] + [C6H11SO3-]). The data for UDT, TDT, and HDT could be fit equally well as first-order and second-order (Table 4; Figure 7). In contrast, χ4(t) data for ODT and ECT could not be fit as either first- or second-order. For case B, the extent of reaction is given by χ5(t) ) [C6H11SO3-]/ ([M-] + [C6H11SO3-]). The χ5(t) data for all SAMs studied can be fit to first-order reactions (Figure 7c; Table 5). We note that χ5(t) data for ODT could also be fit as second-order, with a rate coefficient of k5 ) (2.2 ( 0.1) × 10-2 min-1. The calculated rate constants for the simultaneous oxidation and decomposition of the SAMs are much larger than those for the decomposition of the alkylsulfonate, suggesting that the majority of the decomposition products are formed during the former

Figure 7. (a) Variation of ln χ4′(t) with UV exposure time for UDT, TDT, and HDT adsorbed on GaAs(001). (b) Variation of 1/χ4′(t) with UV exposure time for UDT, TDT, and HDT adsorbed on GaAs(001). (c) Variation of ln χ5′(t) with UV exposure time for UDT, TDT, HDT, ODT, and ECT adsorbed on GaAs(001).

TABLE 5: Calculated Rate Constants for the Decomposition of UDT, TDT, HDT, ODT, and ECT to Form C6H11SO3

SAM

first-order rate constant (min-1) for M C6H11SO3a

UDT TDT HDT ODT ECT

(1.2 ( 0.2) × 10-1 (5.2 ( 0.7) × 10-2 (4.4 ( 0.4) × 10-2 (1.4 ( 0.2) × 10-2 (7.3 ( 1.1) × 10-3

a Uncertainties are obtained from linear regression analysis at 95% confidence.

process. For UDT, TDT, and HDT, once the alkylsulfonate has formed, it may also decompose further on the surface. Taken together, these results indicate that the photooxidation reaction pathway is different for ODT and ECT than for UDT, TDT and HDT. For all monolayers studied the kinetic analyses suggest that SAMs simultaneously decompose and oxidize to form alkylsulfonates and alkylsulfates on the surface. In the case of UDT, TDT, and HDT, once formed, the alkylsulfonate may also oxidize further to form an alkylsulfate or decompose.

Photooxidation of SAMs Adsorbed on GaAs(001)

Figure 8. Extent of photooxidation χ(t) of (a) As-S and (b) Ga-S bonds for UDT, TDT, HDT, ODT, and ECT adsorbed on GaAs(001) as a function of UV exposure time. The lines are the best kinetic fits to the experimental data.

Kinetics of the Photooxidation of the GaAs(001) Substrate. Unlike metal substrates, the GaAs substrate also photooxidizes.47 Initially, in the TOF SIMS mass spectra, we observe both GaS- and As-S-containing ions, for example, 69GaxSy-, 71GaxSy-, and AsxSy-. Upon exposure to UV light, the intensities of AsxSyions rapidly decrease to zero, while the intensities of AsxOyions significantly increase (Figure 1). We can monitor the extent of oxidation of the As-S bonds using χAs-S(t) ) [AsO2-]/ ([AsO2-] + [AsS-]). In Figure 8a, it can be seen that for all of the methyl-terminated SAMs studied except ECT, the photooxidation of the As sites is very fast and is complete within ∼5 min. For ECT, the photooxidation of the As sites is complete after 120 min; χAs-S(t) data can be fit to either first-order or second-order reaction kinetics with rate constants of (4.4 ( 0.2) × 10-2 and (8.7 ( 0.8) × 10-1 min-1. In contrast, the intensities of GaxSy- ions decrease more slowly (Figure 1). To determine the extent of oxidation, χGa-S(t), of the Ga-S bonds, we calculated the extent of oxidation using χGa-S(t) ) [GaO2-]/ ([GaO2-] + [GaS-]). These data clearly show that the photooxidation of the Ga-S bonds is slower for longer-chain alkanethiolates (Figure 8b). For UDT and TDT, χGa-S(t) is consistent with first-order kinetics, while for HDT, the data can be fit to both pseudo-first-order and second-order kinetics (see Supporting Information). For ODT and ECT, the reaction is not well-described by a single first-order rate equation but, rather, is consistent with a second-order rate equation (see Supporting Information). We note that for UDT and TDT, the calculated rate constants are very similar to those observed for the photooxidation of the alkanethiolate, M, to the alkylsulfonate, MSO3, which suggests that the rate-determining step is the diffusion of active oxygen species through these monolayers, in agreement with previous experiments.39 For HDT, ODT, and ECT, the rate constants differ from those observed for the photooxidation of the monolayer, suggesting that the reaction is more complex for these monolayers and is not solely

J. Phys. Chem. C, Vol. 112, No. 3, 2008 803 determined by the rate of diffusion of active oxygen species through the monolayer. 3.4. Adsorbate Packing and Substrate Has a Strong Influence on the Photooxidation Rate. In earlier studies of SAM photooxidation on metals, it was demonstrated that the rate constant for the oxidation reaction increased with decreasing methylene chain length.45,54,55 More recently, it was shown that the rate of photooxidation of UDT on GaAs(001) was faster than that for ODT.39 The present data are in agreement with these observations. The photooxidation of UDT is complete after ∼20 min, whereas for ECT, the reaction takes ∼120 min. One likely reason is that short-chain alkanethiolates adsorbed on GaAs(001) are less ordered, facilitating the penetration of active oxygen species between the alkanethiolate molecules. Recent studies of the structures of methyl-terminated SAMs adsorbed on GaAs(001) support this hypothesis.20,22,48 For alkanethiolates with fewer than 14 carbons in the methylene chain, SAMs exhibit no translational order, a loss of order in the alkyl chain conformation, and a decrease in surface coverage.48 Alkanethiolate SAMs with more than ∼16 carbons in the methylene chain are highly ordered.20,22,48 For methyl-terminated SAMs adsorbed on GaAs(001), the calculated rate constants are 2-4 times larger than those obtained for methyl-terminated SAMs adsorbed on Au under the same experimental conditions (see Supporting Information), which suggests that oxygen penetrates more rapidly to the S/GaAs interface than to the S/Au interface. This is consistent with recent infrared spectroscopy (IRS) data.20,48 For methylterminated SAMs with fewer than 15 carbons in the methylene chain on GaAs(001), the monolayer is disordered, with chain tilts between 0 and 30°.48 In contrast, SAMs adsorbed on Au monolayers are well-ordered, with a chain tilt of ∼30° for alkanethiolates with more than ∼9 carbons.1,2 Thus, we would expect that the penetration of oxygen to the S/GaAs interface is faster for SAMs with fewer than 15 carbons in the methylene chain. However, for methyl-terminated SAMs adsorbed on GaAs(001) with more than 15 carbons, the average tilt angle is 14-15°, which is smaller than the tilt angle reported for Au.20,48 McGuiness et al.48 also observed for methyl-terminated SAMs with greater than 15 carbons that the methylene chains exhibit a high degree of conformational order and are arranged in a herringbone structure similar to that observed for SAMs adsorbed on Au. Thus, one might expect that the photooxidation rate would be similar, or even slightly lower, for longer-chain SAMs adsorbed on GaAs(001) than that for long-chain SAMs on Au, but we do not observe this behaviorsthe photooxidation of SAMs on GaAs(001) is always faster than that for SAMs adsorbed on Au. This observation suggests that for SAMs adsorbed on GaAs(001) with more than ∼16 carbons in the methylene chain, the rate of diffusion of the active oxygen species through the monolayer is only one of several factors that influence the photooxidation rate. A second indication that the photoreaction pathway is different for long-chain and short-chain SAMs adsorbed on GaAs(001) is that we observe a change in the apparent kinetics from first-order to second-order as the chain length increases. Furthermore, the rate of oxidation of the Ga-S bonds deviates from the rate of photooxidation of the SAM, supporting the hypothesis that the rate of SAM photooxidation is not solely determined by the rate of diffusion of the active oxygen species through these monolayers. We note that the observed changes in the reaction pathway occur for SAMs with more than ∼16 carbons in the methylene chain, which is the number required to form well-ordered SAMs on the GaAs(001).48 Grazing

804 J. Phys. Chem. C, Vol. 112, No. 3, 2008 incidence X-ray diffraction (GIXRD) measurements48 show that for ODT on GaAs(001), the domain correlation length is ∼70 Å, which is smaller than that for SAMs adsorbed on Au{111}.1,2 Within the ordered domain structure, the nearest-neighbor (NN) adsorbate spacings (4.70 and 5.02 Å) are strongly mismatched with the NN and next NN spacings (3.995 and 5.65 Å, respectively) of the GaAs(001) substrate. In order to accommodate this mismatch, it is likely that the substrate undergoes some reconstruction, but the very small domain sizes observed indicate that this reconstruction is probably incomplete.48 These small SAM domain sizes indicate a large concentration of domain boundaries, which may account for the observed higher photooxidation rates since they provide an easy route for oxygen to reach the GaAs/S interface. As the number of methylene units in the SAM increases, it is likely that the domain size will also increase, reducing the oxygen mobility. A second important consequence of the methylene chain packing forcing a reconstruction of the underlying GaAs(001) substrate is that, as the photoreaction proceeds, the monolayer will become damaged, and the driving force for the surface reconstruction will be lessened. Thus, during the reaction, it is likely that the GaAs(001) substrate will reconstruct, which may affect the reaction kinetics. These two effects may cause the observed switch from first-order to second-order kinetics with increasing methylene chain length as well as the observed deviation of the rate of oxidation of the Ga-S bonds from the rate of photooxidation of the SAM. Further work is required to examine the role of the GaAs(001) substrate structure in the SAM photooxidation reaction. 4. Conclusions We have investigated the reaction pathways involved in the UV photooxidation of a series of methyl-terminated alkanethiolate SAMs adsorbed on GaAs(001). We observe that for UDT and TDT, the reaction kinetics are well-described by pseudofirst-order rate constants. For HDT, ODT, and ECT, there is a change in the apparent kinetics from first-order to second-order. In agreement with previous studies of the UV photooxidation of SAMs adsorbed on metals,45,54,55 the rate of photoreaction is determined by the penetration of oxygen to the GaAs/S interface for short-chain alkanethiols (n < 16). However, for longer-chain alkanethiols, the photooxidation rate is not solely determined by the penetration of the active oxygen species to the surface. Unlike the case of SAMs adsorbed on metals, the GaAs substrate also photooxidizes upon UV exposure. The kinetic analyses suggest that SAMs adsorbed on GaAs(001) simultaneously photooxidize to form alkylsulfonates and alkylsulfates as well as decompose. In the case of UDT, TDT, and HDT, once formed, alkylsulfonates may also oxidize further to form alkylsulfates or decompose. This behavior contrasts with that for SAMs adsorbed on Au and Ag,36-38,46,54,55 which initially oxidize to form a sulfonate. The photooxidation rates for SAMs adsorbed on GaAs(001) are faster than those for SAMs adsorbed on metals. Short-chain alkanethiols (n e 15) do not form well-ordered monolayers on GaAs(001),20,22,48 and therefore, the penetration of the active oxygen species to the S/substrate interface is faster than that for SAMs which form well-ordered monolayers on metals.1,2 However, for longer-chain alkanethiols (n g 16), which form well-ordered monolayers on GaAs(001), the photooxidation rate is not solely determined by the rate of penetration of oxygen through the monolayer. Well-ordered SAMs adsorbed on GaAs(001) have much smaller domain sizes on GaAs(001) than those on Au{111}.20,22,48 These small domains may be due to a partial

Zhou and Walker reconstruction of the GaAs(001) driven by the methylene chainpacking forces.48 Small domains correspond to a high concentration of domain boundaries and may account for the higher photooxidation rate. Furthermore, during the photooxidation reaction, the monolayer structure becomes damaged, and the driving force for the reconstruction is lessened. This may lead to reconstruction of the GaAs(001) substrate during the reaction. These factors may cause the observed change from first-order to second-order kinetics with increasing chain length, as well as the deviation of the rate of the oxidation of the Ga-S bonds from the rate of photooxidation of the SAM. Acknowledgment. The authors would like to acknowledge the financial support of a National Science Foundation grant (Grant No. ECS-506802). Supporting Information Available: TOF SIMS negative ion mass spectra of UDT, HDT, ODT, and ECT adsorbed GaAs(001) upon exposure to UV light; calculation of the rate coefficients for the extent of Ga-S degradation; TOF SIMS negative ion mass spectra of ODT adsorbed on Au upon exposure to UV light; and calculation of the rate coefficients for the extent of photooxidation of UDT, TDT, HDT, and ODT adsorbed on Au. This material is available free of charge on the Internet at http://pubs.acs.org. References and Notes (1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (2) Ulman, A. Chem. ReV. 1996, 96, 1533. (3) Kirchner, C.; George, M.; Stein, B.; Parak, W. J.; Gaub, H. E.; Seitz, M. AdV. Funct. Mater. 2002, 12, 266. (4) Nesher, G.; Vilan, A.; Cohen, H.; Cahen, D.; Amy, F.; Chan, C.; Hwang, J.; Kahn, A. J. Phys. Chem. B 2006, 110, 14363. (5) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Martensson, J.; Allara, D. L. Jpn. J. Appl. Phys., Part 1 1991, 30, 3759. (6) Janes, D. B.; Lee, T.; Liu, J.; Batisuta, M.; Chen, N.-P.; Walsh, B. L.; Andres, R. P.; Chen, E.-H.; Melloch, M. R.; Woodall, J. M.; Reifenberger, R. J. Electron. Mater. 2000, 29, 565. (7) Hsu, J. W. P.; Loo, Y. L.; Lang, D. V.; Rogers, J. A. J. Vac. Sci. Technol., B 2003, 21, 1928. (8) Hsu, J. W. P.; Lang, D. V.; West, K. W.; Loo, Y.-L.; Halls, M. D.; Raghavachari, K. J. Phys. Chem. B 2005, 109, 5719. (9) Li, W.; Kavanagh, K. L.; Matzke, C. M.; Talin, A. A.; Leonard, F.; Faleev, S.; Hsu, J. W. P. J. Phys. Chem. B 2005, 109, 6252. (10) Lodha, S.; Carpenter, P.; Janes, D. B. J. Appl. Phys. 2006, 99, 024510. (11) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476. (12) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1994, 12, 3663. (13) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139. (14) Ramashan, K.; Bhat, K. N. Thin Solid Films 1999, 342, 20. (15) Blakemore, J. S. J. Appl. Phys. 1982, 53, R123. (16) Sheen, C. W.; Shi, J.-X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (17) Ye, S.; Li, G.; Noda, H.; Uosaki, K.; Osawa, M. Surf. Sci. 2003, 529, 163. (18) Dorsten, J. F.; Maslar, J. E.; Bohn, P. W. Appl. Phys. Lett. 1995, 66, 1755. (19) Adlkofer, K.; Tanaka, M.; Hillebrandt, H.; Wiegand, G.; Sackmann, E.; Bolom, T.; Deutschmann, R.; Abstreiter, G. Appl. Phys. Lett. 2000, 76, 3313. (20) McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zharnikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231. (21) Jun, Y.; Zhu, X.-Y.; Hsu, J. W. P. Langmuir 2006, 22, 3627. (22) McGuiness, C. L.; Shaporenko, A.; Zharnikov, M.; Walker, A. V.; Allara, D. L. J. Phys. Chem. C 2007, 111, 4226. (23) Adlkofer, K.; Eck, W.; Grunze, M.; Tanaka, M. J. Phys. Chem. B 2003, 107, 587. (24) Adlkofer, K.; Shaporenko, A.; Zharnikov, M.; Grunze, M.; Ulman, A.; Tanaka, M. J. Phys. Chem. B 2003, 107, 11737. (25) Krapchetov, D. A.; Ma, H.; Jen, A. K. Y.; Fischer, D. A.; Loo, Y.-L. Langmuir 2005, 21, 5887.

Photooxidation of SAMs Adsorbed on GaAs(001) (26) Donev, S.; Brack, N.; Paris, N. J.; Pigram, P. J.; Singh, N. K.; Usher, B. F. Langmuir 2005, 21, 1866. (27) Cho, Y.; Ivanisevic, A. J. Phys. Chem. B 2005, 109, 12731. (28) Adlkofer, K.; Tanaka, M. Langmuir 2001, 17, 4267. (29) Yang, G. H.; Zhang, Y.; Kang, E. T.; Neoh, K. G.; Huang, W.; Teng, J. H. J. Phys. Chem. B 2003, 107, 8592. (30) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823. (31) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1171. (32) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (33) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (34) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F., Jr. Anal. Chem. 1994, 66, 2170. (35) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024. (36) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089. (37) Zhou, C.; Walker, A. V. Langmuir 2006, 22, 11420. (38) Brewer, N. J.; Janusz, S.; Critchley, K.; Evans, S. D.; Leggett, G. J. J. Phys. Chem. B 2005, 109, 11247. (39) Zhou, C.; Walker, A. V. Langmuir 2007, 23, 8876. (40) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414.

J. Phys. Chem. C, Vol. 112, No. 3, 2008 805 (41) Sun, S.; Leggett, G. J. Nano Lett. 2002, 2, 1223. (42) Sun, S.; Leggett, G. J. Nano Lett. 2004, 4, 1381. (43) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (44) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656. (45) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (46) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (47) Hollinger, G.; Skheyta-Kabbani, R.; Gendry, R. Phys. ReV. B 1994, 49, 11159. (48) McGuiness, C. L.; Blasini, D.; Masejewski, J. P.; Uppili, S.; Cabarcos, O. M.; Smilgies, D.; Allara, D. L. ACS Nano 2007, 1, 30. (49) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (50) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (51) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (52) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (53) ToF SIMS: Surface Analysis by Mass Spectrometry; Vickerman, J. C., Briggs, D., Eds.; IM Publications and Surface Spectra Limited: Chichester, U.K., 2001. (54) Cooper, E.; Leggett, G. J. Langmuir 1998, 14, 4795. (55) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174. (56) Bordoli, R. S.; Vickerman, J. C. Surf. Sci. 1979, 85, 244. (57) Brown, A.; Vickerman, J. C. Vacuum 1981, 31, 429.