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Photochemical Grafting of Alkenes onto Carbon Surfaces: Identifying the Roles of Electrons and Holes Xiaoyu Wang,† Paula E. Colavita,† Jeremy A. Streifer,† James E. Butler,‡ and Robert J. Hamers*,† Department of Chemistry, UniVersity of WisconsinsMadison, 1101 UniVersity AVenue, Madison, Wisconsin 53706, and NaVal Research Laboratory, Code 6174, Washington, DC 20375 ReceiVed: NoVember 27, 2009; ReVised Manuscript ReceiVed: January 30, 2010
We report a mechanistic investigation of the charge transfer processes that occur during photochemical grafting of liquid alkenes to H-terminated surfaces of diamond and amorphous carbon. Spectrally resolved photoelectron yield experiments were performed to directly characterize the photoemission of electrons from the hydrogenterminated surfaces into liquid alkenes, using trifluoroacetamide-protected 1-aminodec-1-ene (TFAAD) and 10-N-Boc-aminodec-1-ene (tBoc) as model alkenes having different terminal acceptor groups; 1-dodecene was also used as a control. Corresponding X-ray and ultraviolet photoelectron spectroscopy measurements (XPS, UPS) establish a clear correlation between the photoelectron yield, the grafting efficiency at different wavelengths, and the valence electronic structure of the substrate and of the reactant molecule. Direct imaging of the molecular layers via scanning electron microscopy shows that there are substantial differences in the sharpness of molecular patterns that can be produced on single-crystal type Ib (low-mobility) and type IIb (high-mobility) diamond samples. Our results demonstrate that electrons and holes both play important and distinct roles in the photochemical grafting of alkenes to diamond and amorphous carbon surfaces. Introduction Photochemically induced grafting of alkenes to surfaces has emerged as an effective way to modify the chemical and physical properties of semiconductors.1-11 Recent studies have shown that ultraviolet (UV) light-induced grafting is effective on H-terminated carbon surfaces including metallic (graphitic) forms such as carbon nanofibers12,13 and glassy carbon14 and semiconducting forms such as amorphous carbon15 and diamond.5-7,16-18 While UV-induced grafting is versatile, it has remained unclear whether there is a single mechanism or whether different combinations of substrates and molecules may react via different reaction pathways.2,7,16,19-21 The photochemically induced grafting of alkenes to diamond is especially intriguing because the 4.9 eV (254 nm) photon energy typically used is below the 5.5 eV bulk bandgap of diamond and in an optical region where the organic reactant liquids as well as the diamond substrate are nearly transparent (see the Supporting Information). Recently, we showed that UV light can induce photoemission of electrons from diamond into adjacent reactant liquids,7,21 and studies on diamond and amorphous carbon suggested that this “internal photoemission” process initiates the grafting reaction.7,15,22,23 However, a number of key questions remain unresolved. While each photoemission event creates a liquid-phase anion and an accompanying hole in the diamond sample, it is unknown what role the anion and hole play in the subsequent grafting reaction.7 Photoemission at sub-bandgap wavelengths can arise from several different pathways,24 leaving open many questions about how the electronic properties of the substrate and substrate-molecule interface influence the reaction. Here, we report experiments aimed at elucidating the distinct roles that the photoemitted electrons and the resulting holes play * To whom correspondence should be addressed. † University of WisconsinsMadison. ‡ Naval Research Laboratory.
in controlling the photochemical grafting of alkenes to surfaces of carbon. We report the first spectrally resolved photoelectron yield measurements of diamond and amorphous carbon in contact with organic liquids, and we correlate the wavelength dependence of photoelectron yield with the efficiency of photochemical grafting characterized by X-ray photoelectron spectroscopy (XPS). These measurements directly establish the role of photoelectron emission in initiating reactivity toward organic alkenes. We also use photopatterning methods to study how the efficiency and distribution of molecules are affected by changes in hole mobility using type Ib (low-mobility) and type IIb (high-mobility) diamonds. These studies provide new insights into the photochemical grafting of alkenes on surfaces and establish the distinct but complementary roles of excited electrons and holes in this process. While demonstrated here on carbon surfaces, the results are expected to have more general implications for understanding the mechanism of photochemical grafting of alkenes on other surfaces, especially other widebandgap semiconductors.9,10 Experimental Section Chemicals. The experiments reported here were conducted using two alkenes: trifluoroacetic acid protected 10-aminodec1-ene (TFAAD, Almac Sciences) and 10-N-Boc-amino-dec-1ene (tBoc, Astatech, Inc.). Some experiments were also performed using 1-dodecene as a control. TFAAD and tBoc are both depicted in Figure 1a. Both molecules were purified by vacuum distillation. Chloroform (Fisher) and electronic grade methanol (Fisher) were used without further purification. Substrate Preparation. The experiments reported here used both single-crystal and nanocrystalline diamond samples. For photocurrent measurements, nanocrystalline diamond (NCD) films (p-type, B-doped, 1018 dopants/cm3, ∼0.5 µm thick) grown on fused silica substrates were used. Photopatterning studies used single-crystal type IIb diamond with cleaved (111)-oriented
10.1021/jp911264n 2010 American Chemical Society Published on Web 02/12/2010
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Figure 1. (a) Molecules used in the studies reported here. (b) Experimental cell setup for photocurrent measurement. (c) Experimental setup of in situ internal photoemission spectroscopy.
top and bottom faces and type Ib diamond with (100)-oriented top and bottom faces (Sumitomo HPHT diamond). The type Ib diamond was determined to have a N concentration of 24 ppm (∼4 × 1018 cm-3) based on optical absorption measurements at 400 nm.25 XPS data were obtained on single-crystal samples, nanocrystalline thin films (p-type, B-doped, ∼0.5 µm thick) grown on silicon (111) wafers grown on silicon (111) wafers. Previous studies showed that grafting also occurred on n-type (N-doped) samples.26 Amorphous carbon (a-C) samples for photocurrent measurements were fabricated using lithography techniques to produce interdigitated electrodes (0.01 in. wide, with 0.01 in. spacing between each finger) consisting of a 30 nm film of Ti (produced by electron-beam deposition) followed by a 100 nm film of sputter-deposited amorphous carbon. All diamond and amorphous carbon samples were hydrogen terminated before use in a 13.56 MHz RF hydrogen plasma. Following the H-termination, Au thin films (∼30 nm thick) were sputtered onto the sample edges to produce ohmic contacts. Photocurrent Measurement. For nanocrystalline diamond films (on fused silica substrates), the configuration in Figure 1b was used. The diamond sample was used as the top electrode with the diamond side facing the liquid, and Pt foil was used as the bottom electrode. A Teflon spacer (0.003 in. thick) with a 0.25 in. diameter hole provided a reservoir for the liquid and also provided a defined area with constant electrode separation. A small weight (22 g) with a 0.225 in. diameter aperture was placed on top of the diamond electrode. Photoelectron yield experiments on diamond (Figure 1c) used a 450 W Xe arc lamp focused onto the entrance slit of a monochromator (Acton SpectraPro-275 with a 600 grooves/mm grating blazed at 300 nm) using a CaF2 lens. Another CaF2 lens and a mirror were placed behind the exit slit of the monochromator to slightly focus the light onto the sample, forming an
Wang et al. illuminated region approximately 3 mm × 3 mm. A beam shutter (Uniblitz, LS6 with VMM-D4 controller) was placed at the entrance slit of the monochromator. Both direct-current (DC) and phase-sensitive photocurrent measurements were performed. DC photocurrent measurements were made using a Keithley 2400 Sourcemeter to measure the current in the absence and presence of light under conditions of fixed voltages (typically ∼40 mV or less) applied to the diamond. Phase-sensitive measurements (as depicted in Figure 1c) were made using a shutter driven at 0.5 Hz and measuring the resulting photocurrent using a dual-phase lock-in amplifier (Stanford Research System, SR830). Because amorphous carbon is not transparent at 254 nm, measurements on a-C samples used a planar set of interdigitated amorphous carbon electrodes on a fused silica substrate (Figure 4c, inset; detailed illustration in the Supporting Information). A small amount of the organic liquid (TFAAD or tBoc) was placed on the electrodes and then covered with a fused silica window to reduce evaporation. The assembly was placed inside a nitrogen-purged reaction chamber with a quartz window and illuminated. A DC bias of 40 mV was applied and the current measured using a lock-in amplifier synchronized to the electronic shutter. Photochemical Grafting and Patterning. A small droplet of reactant alkene was placed onto the sample, and a fused silica window was placed directly on top of the liquid film, trapping a film of reactant liquid. The sample and cover were placed inside a nitrogen-purged reaction chamber and illuminated with light of the appropriate wavelength. In patterning experiments, electron-beam lithography and electron-beam deposition methods were used to pattern chromium rectangles onto a fused silica window; this was then placed directly onto the liquid-covered samples and illuminated as above. Wavelength-dependent measurements used the Xe lamp with a monochromator, while patterning experiments used 254 nm light from a low-pressure mercury lamp (254 nm, ∼10 mW/cm2). After the reaction, the diamond samples were sonicated in chloroform twice (5 min each), followed by electronic grade methanol (5 min), and dried with N2 before analysis. Surface Characterization. The samples were characterized using X-ray photoelectron spectroscopy (XPS) in an ultrahigh vacuum system (P < 5 × 10-10 Torr) equipped with a loadlock for sample introduction, a monochromatized Al KR source (1486.6 eV), and a hemispherical analyzer with a multichannel detector. The photopatterned diamond samples were imaged using a Leo/Zeiss Supra55 VP scanning electron microscope (SEM). Data shown here were obtained using 2 kV incident electron energy, using the standard in-lens detector. Results Direct Measurement of Photoemission Current from Diamond into Liquid Alkenes. While the photoinduced current between two carbon electrodes was measured previously,15,21 in such a symmetric design, the oxidation and reduction processes occurring at the two electrodes cannot be easily separated. Therefore, we used the asymmetric cell depicted in Figure 1b, in which the diamond sample was coupled to a platinum electrode. Pt was chosen because its high work function27 of 5.65 eV minimizes the possibility of having photoemission from Pt at the wavelengths used here. Figure 2 shows the steady current profiles when a Hterminated NCD diamond in contact with TFAAD is illuminated with UV light. Data are shown with bias voltages of +40 mV (trace A), 0 V (trace B), and -40 mV (trace C) applied to the
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Figure 3. Energy level diagram showing boron doped p-type NCD valence and conduction band edges (VBM and CBM, respectively) at surfaces and the Pt work function.
Figure 2. Current through a diamond-TFAAD-platinum electrode assembly in the presence and absence of 254 nm UV light, at different DC bias voltages applied to the diamond sample. “On” and “off” indicate when the 254 nm light was turned on and off. Measurements are shown with the following bias voltages applied to the diamond sample: (a) +40 mV; (b) no bias; (c) -40 mV.
diamond sample. No beam shutter or lock-in amplifier was used in these experiments. The sourcemeter sign convention is such that positive current corresponds to positive charges moving from diamond through the liquid to the Pt electrode. In the absence of illumination, a small background current of 1 nA is observed when no bias is present; this increases to ∼ +3.5 nA with an applied voltage of 40 mV and switches sign, to -1.5 nA, when a negative voltage is applied. In all three cases, illumination with 254 nm UV light causes the current to shift immediately (limited by the ∼2 s time response of the measurement system) in the negative direction by ∼1 nA. When illumination is stopped, the current quickly recovers to its dark value. The difference between the photocurrent and the baseline dark current remains constant throughout the illumination duration (typically ∼10 min), indicating the electrochemical properties of the electrode surfaces and TFAAD liquid are not significantly altered during the measurement time used in these experiments. To establish the positions of the relevant energy levels, we performed ultraviolet photoemission (UPS) measurements on H-terminated diamond. We have previously reported similar UPS measurements on nearly identical samples,7 and here include only a representative spectrum in the Supporting Information. For H-terminated, B-doped diamond, our measurements show that the Fermi level at the surface is 0.54 eV above the valence band maximum (VBM) and the vacuum level lies 4.36 eV above the Fermi level. Using the known bandgap of 5.48 eV28 at 295 K, this places the vacuum level 0.58 eV below the conduction band minimum. These measurements confirm the negative electron affinity (χ) of diamond29 (χ ) -0.58 eV) and all values are in good agreement with previous studies.7,30,31 At equilibrium (with no bias), the Fermi energy of diamond aligns with that of the Pt. Figure 3 depicts the resulting band alignment of diamond (B-doped, p-type) and Pt. Since polycrystalline Pt has a work function of 5.64 eV,28 the vacuum level of Pt is therefore ∼1.3 eV higher than that of diamond. The resulting contact potential between diamond and Pt is expected to produce an electric field in the liquid alkene oriented such that any charge carriers created by ionization within the liquid will produce a current that is positive, as measured in Figure 2. Therefore, the fact that illuminating the sample causes the current to shift in the negative direction, independent of DC bias, establishes that the photocurrent arises from negative
charge carriers moving through the reactant liquid from diamond toward Pt. Since the data in Figure 2 show that a bias voltage is not necessary to detect photoelectron emission in the asymmetric NCD/organic liquid/Pt cell setup, the remaining experiments described below were performed with no external bias. Spectrally Resolved Photoemission into Alkenes. In order to understand the influence of wavelength on the physical processes that induce grafting, we measured the photoemission yield of H-terminated diamond in contact with TFAAD and tBoc as a function of photon energy and measured the lamp emission spectrum using a calibrated pyroelectric detector (SPH-20, Spectrum Detector, Inc.). Normalizing the photoemission current by the lamp intensity and the photon energy Ephoton yield the photon yield spectra presented in Figure 4a; these curves are proportional to the photoelectron yield per incident photon. The data in Figure 4a were obtained scanning the monochromator from 400 to 200 nm; identical results were obtained scanning in the opposite direction, demonstrating that the sample is not significantly altered by exposure to light on the time scale of this experiment. The data for TFAAD shows a clear threshold near Ephoton ) 4.0 eV. At higher energies, the yield spectrum can be divided into two regions: the yield rises slowly between 4.0 and 5.5 eV and then increases more rapidly for Ephoton > 5.5 eV. Notably, at all wavelengths, the photoelectron yield at the diamond-tBoc interface is lower than that of the diamond-TFAAD interface. We also attempted to perform photoemission yield experiments using 1-dodecene, which does not have a terminal acceptor group; however, no detectable current could be observed. This confirms that photoemission into TFAAD and tBOC involves the terminal acceptor group and not the vinyl group. Prior studies have shown that the yield (Y) for photoemission of electrons from semiconductors into vacuum32,33 and into liquids such as water34,35 often follows a power-law dependence, Y ≈ A(Ephoton - Ethreshold)γ. In this equation, Ethreshold is a threshold energy corresponding to the difference in energy between the semiconductor valence band and the energy of the electron after emission. The prefactor A is a constant of proportionality, and the exponent γ depends on the dimensionality of the electronic states involved and momentum conservation rules but typically ranges from 3/2 to 5/2, with higher values characteristic of indirect excitations (either bulk or surface) in which the electron momentum parallel to the surface is not conserved.32-35 We attempted to fit the sub-bandgap photoelectron yield data to a function of this form, with results shown in the Supporting Information. The TFAAD data fits a power law well for 4.1 eV < Ephoton < 4.6 eV, yielding γ ) 2.47 and Ethreshold ) 3.92 V. Similar attempts for tBoc did not yield acceptable fits, and the low-energy region is closer to exponential in
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Figure 4. (a) Spectrally resolved photoelectron yield of H-terminated NCD in contact with TFAAD (red) and tBoc (blue), presented as yield per incident photon, in arbitrary units. (b) Extent of reaction of TFAAD with H-terminated NCD at different wavelengths. The measurements at 254, 275, and 290 nm were done at a constant fluence (F); the reaction using 360 nm light was carried out with a much larger fluence to confirm the presence of a cutoff. (c) Photoelectron yield spectrum of H-terminated amorphous carbon in contact with TFAAD, presented as yield per incident photon, in arbitrary units. The inset depicts the interdigitated electrode array used.
character. While a clear threshold is difficult to identify for tBoc, in the 4.0-5.0 eV region, the curve for tBoc is shifted ∼0.7-0.8 eV toward higher energy compared with that of TFAAD, suggesting a threshold of ∼4.7 eV for photoemission into tBOC. At the 4.88 eV energy (λ ) 254 nm) commonly used in previous grafting studies,6,7,16,17,21 the photoemission yield using TFAAD is ∼5 times higher than that observed using tBOC. Wavelength Dependence of Alkene Grafting to H-Terminated Diamond. To test whether the photoemission yield and the grafting process follow similar trends, we investigated the grafting of TFAAD to NCD at three different wavelengths: 254, 275, and 290 nm, corresponding to photon energies of 4.9, 4.5, and 4.3 eV. To characterize the relative rates of grafting rather than the saturation coverage, we used a relatively short reaction time of 2 h for the 254 nm light as the standard reaction conditions; we have previously shown that the amount of grafting is linear in time in this short time.7 The illumination times for the 275 and 290 nm were decreased to 56 and 42 min, respectively, so that the same total fluence (total number of photons) reached the diamond surface in each experiment. Control samples showed no reaction of diamond with TFAAD or other alkenes in the absence of short wavelength light. TFAAD can undergo slow electron-induced
Wang et al. decomposition when illuminated at 254 nm at long times,15,22,23,36 but at the short times used here there is no significant degradation. Consequently, the extent of reaction was characterized by the peak area ratio (without sensitivity factor correction) AF(1s)/AC(1s) in the XPS spectra. Figure 4b shows the results for TFAAD: The AF(1s)/ AC(1s) ratios were 0.086, 0.068, and 0.012 for the samples reacted using 254 nm (4.88 eV), 275 nm (4.51 eV), and 290 nm (4.28 eV) light, respectively. The photon yields at these three wavelengths are in a ratio of Y254nm:Y275nm:Y290nm ) 1.0:0.44:0.20. These data show that the grafting reaction and the photoemission yield exhibit similar behavior as the excitation wavelength is decreased from 254 to 290 nm. As an additional control, we also investigated whether photon energy lower than the photoemission threshold of ∼4.0 eV inferred from the spectrally resolved photoemission studies could induce photochemical reaction at higher illumination intensities. The TFAAD-covered H-terminated diamond surface was illuminated with 360 nm (3.5 eV) light for 2.5 h; even though the photon flux is 7 times higher and the illumination time slightly longer than those used in the 254 nm measurements, XPS analysis of this sample yielded a AF(1s)/AC(1s) ratio of only 0.009, which is not distinguishable from the noise level. From these data, we conclude that there is a well-defined threshold energy below which grafting does not occur, and that this energy corresponds to the energy required to photoemit electrons from diamond into the adjacent alkene. Comparison with Photoemission Yield on Amorphous Carbon. Alkenes will also graft to H-terminated surfaces of amorphous carbon, with TFAAD exhibiting higher reactivity than tBoc.15 Figure 4c shows the photoelectron yield spectrum of H-terminated amorphous carbon in contact with TFAAD. The onset of the photoemission decreases to ∼3.5 eV, 0.5 eV lower in energy than that of diamond. The photoelectron yield increases gradually as the photon energy is increased from 3.5 to 5.0 eV, levels off, and then rises sharply beyond ∼5.7 eV. We also obtained UPS data for H-terminated amorphous carbon (not shown);15 from the UPS data, we obtained a work function of 4.0 eV and found the valence band edge lies 0.5 eV below EF; thus, the valence band of amorphous carbon lies ∼4.5 eV below the vacuum level, while the valence band of diamond lies 4.90 eV below the vacuum level. The 0.5 eV reduction in the photoemission energy threshold for amorphous carbon compared with diamond is in excellent agreement with the shift expected from the difference valence band positions, further supporting the connection between the photoemission threshold, and with the energy of the valence band edge of the substrate. This strongly suggests that the internal photoemission process dominates the photochemical reaction on both diamond and amorphous carbon surfaces. Influence of Bulk Mobility of Spatial Distribution and Efficiency of Grafting. The above experiments demonstrate that grafting is initiated by photoelectron emission into the alkene but do not address whether subsequent chemical steps in the grafting reaction involve the anions in the liquid phase or the holes that are produced in the diamond. To address this question, we used photopatterning methods to produce a sharp mask that allowed UV light to strike only half the sample, and we characterized the distribution of molecules grafted to the surface near the boundary. Experiments were conducted using two types of single-crystal diamond having very different hole mobilities. Type IIb diamond is a p-type semiconductor with a high carrier hole mobility, (∼1200 cm2 V-1 s-1 at 300 K),37,38 while in type Ib diamond substitutional nitrogen leads to much lower hole mobility (typically ∼50 cm2 V-1 s-1 at 300 K).30,39 Figure 5 shows SEM images and the corresponding SEM intensity
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Figure 6. XPS data from TFAAD-grafted single-crystal diamond samples. The C (1s) peaks are scaled to be of equal height. (a) Type Ib single-crystal diamond. (b) Type IIb single-crystal diamond.
Figure 5. SEM images of molecular patterns produced after patterning type IIb and Ib single-crystal diamond surfaces with TFAAD, using a sharp mask that prevented light from reaching the left half of each sample: (a) SEM image of type IIb diamond; (b) SEM intensity profile from part a; (c) SEM image of type Ib diamond; (d) SEM intensity profile of type Ib diamond from part c.
profiles (linear scale) from a type IIb sample (Figure 5a,b) and a type Ib sample (Figure 5c,d). Both samples were completely covered with TFAAD and placed under the 254 nm lamp, but an opaque mask prevented the 254 nm light from reaching the left half of each sample. The samples were then rinsed and imaged under identical conditions. In the SEM images, dark regions are TFAAD-functionalized and bright regions are H-terminated. The transition from nonfunctionalized to functionalized regions occurs within a distance of 2 µm for type Ib diamond, while, for type IIb diamond, this width is ∼13 µm, nearly 7 times wider than that observed on the type Ib sample. The much larger transition width on type IIb diamond compared with type Ib diamond has been repeated on multiple singlecrystal samples with equivalent results. To verify that these Ib and IIb diamonds have similar efficiencies of grafting, we used XPS to compare the molecular coverage after TFAAD-covered samples were illuminated with 254 nm light for 3 h; this time was chosen to be in the linear range, before selfterminating behavior is observed.7 Figure 6 shows the resulting
XPS survey spectra of type Ib (Figure 6a) and type IIb (Figure 6b) samples. Quantitative measurements of the F(1s) and C(1s) peak areas in high-resolution spectra (not shown) yield a peak area ratio of AF(1s)/AC(1s) ) 0.44 for the type Ib sample and AF(1s)/AC(1s) ) 0.61 for the type IIb sample. The difference between these values is small, indicating that TFAAD grafts at a similar rate on both Ib and IIb samples. The type Ib sample has a nitrogen content of ∼4 × 1018 cm-3, giving rise to a weak absorption of 0.12 (at 254 nm) in a 1 mm thick sample. While at much higher concentrations N-derived defects can overlap to form a band of states, at the concentration present in our samples, the N-derived states are effectively isolated.40,41 Due to the low concentration of nitrogen and the fact that the grafting rate is similar on type Ib and IIb samples, we conclude that excitations involving these states do not play a significant role in the grafting process under the conditions explored here. The data in Figure 5 show that changes in hole mobility strongly affect the spatial distribution of molecules, while Figure 6 shows that the bulk mobility has only a minimal effect on the overall grafting efficiency on uniformly illuminated samples. The pronounced difference between type Ib and type IIb diamond samples in Figure 5 implies that the liquid-phase anions produced by photoemission are not responsible for controlling the overall spatial distribution of molecules, and that these anions are likely only spectators with little or no direct role in the subsequent chemistry. Instead, we conclude that the spatial distribution of molecules is controlled by diffusion of the holes that remain in the diamond after the electron photoemission. Discussion While the fact that organic alkenes will graft to surfaces of semiconductors such as silicon2,3,8,20 and diamond6,7 is well established, the mechanisms involved are not well understood. Early studies on silicon proposed an exciton mechanism in which visible light creates electron-hole pairs, and the positive holes facilitate reaction with the electron-rich alkenes.4,42 In the case of diamond, however, UV light at 254 nm (4.88 eV) is not sufficient to excite across the 5.48 eV bandgap to create bulk excitons. On both diamond and amorphous carbon, there are large differences in reactivity between different molecules that differ only in the functional group at the distal end of the molecule that are not readily explained by an exciton model.15,22,23,43 On these materials, we showed that a new initiation pathway occurs in which electrons are directly photoemitted from the substrate into the adjacent reactant fluid,
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Figure 7. Processes leading to internal photoemission at the diamond-alkene interface: (a) Below-bandgap excitation: transition from midgap states (bulk or surface) to conduction band, followed by diffusion and electron emission into alkene. (b) Below-bandgap excitation: direct emission from valence band to liquid alkene acceptor state, possibly mediated by normally unoccupied C-H surface states.
and that specific terminal groups (notably, the trifluoroacetamide group of TFAAD and, to a lesser extent, the tert-butyloxycarbonyl group of tBoc) can enhance the reactivity by acting as electron acceptors.7,15,21,22,43 However, it has remained unclear whether the anions play a direct role in the reaction, or whether the reaction is controlled by the holes that are left behind in the diamond as a result of the photoemission process. The experiments reported here provide new insights into the nature of photochemical grafting on diamond and may significantly impact the mechanistic understanding of grafting on other materials as well. Sub-Bandgap Photoelectron Ejection. The first question to be addressed is how electrons are photoemitted from diamond to the alkenes when using sub-bandgap light. Previous studies have shown that the surface C-H bonds of H-terminated diamond induce negative electron affinity (NEA),17,29,44 such that the vacuum level is lower in energy than the conduction band. In this situation, it is energetically more favorable to directly photoemit electrons into a vacuum or an adjacent dielectric medium than to excite across the bandgap. Sub-bandgap photoemission in diamond can arise in two ways. One pathway (Figure 7a) involves exciting electrons from filled midgap states (e.g., impurity states, defects) to the conduction band, followed by diffusion of electrons to the surface and emission into the adjacent liquid. A second pathway (Figure 7b) occurs by direct excitation of electrons from filled valence states at the surface or possibly from midgap surface states directly into empty acceptor states of the immediately adjacent molecules. Previous studies have shown that H-termination of diamond introduces an unoccupied surface state lying near the vacuum level,45-48 which might mediate this process. Substitutional nitrogen impurities also form isolated defect states lying 1.7 eV below the conduction band.49 While at very high concentrations N-derived states enhance electron field emission, their effect is only observed at high concentrations.50 We rule out the role of N-derived midgap states in our studies because single-crystal type Ib and IIb samples graft at similar rates and both have N concentrations below the concentration where defect-derived conductivity occurs.40,41 If excitation occurred via the diamond conduction band (as in Figure 7a), then all molecules having acceptor levels below the diamond conduction band should have photoelectron yield spectra with an identical wavelength dependence corresponding to that of the bulk absorption process. In contrast, Figure 4 shows that tBoc and TFAAD have photoelectron yield spectra differing
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Figure 8. Energy level diagram showing boron-doped p-type diamond (nanocrystalline diamond and type IIb single-crystal diamond) and amorphous carbon (valence band and Fermi energy shown) obtained from UPS masurements. Also shown are the energies of the alkene acceptor levels inferred from the photoemission thresholds and previously estimated values based on DFT calculations.15
substantially in overall shape and with thresholds differing by ∼0.7 eV, while photoemission into 1-dodecene could not be detected at all. We therefore conclude that the excitations occur from the valence band directly into the molecular acceptor levels via the process shown in Figure 7b. One consequence of this is that it allows assignment of relevant energy levels based on the photoemission data and photoelectron yield thresholds.34 Figure 8 shows the energy levels of diamond and amorphous carbon based on the UPS data. Figure 8 also shows the alkene acceptor levels inferred from the photoelectron yield measurements presented here (“this work”) and, for comparison, the acceptor levels obtained from a previous density functional theory (DFT) calculation.15 The 4.0 eV photoemission threshold for TFAAD indicates that the TFAAD acceptor level lies 4.0 eV above the diamond valence band maximum (VBM); similarly, the tBoc acceptor level lies 4.7 eV above the diamond VBM. Since no photoemission current is observed using 1-dodecene, we can only infer a lower bound, that the 1-dodecene acceptor level lies >4.9 eV above the diamond valence band edge. The DFT predicted values15 in Figure 8 were based on DFT calculations of the total energies of neutral and anionic molecules and then using an approximate Born shift to account for the dielectric properties of the surrounding liquid. While the experimental and calculated acceptor levels are slightly different, the ordering of the molecular acceptor levels is the same, and the calculations agree with our observation that photoelectrons are most easily ejected into TFAAD, slightly less readily into tBoc, and only with great difficulty (not detectable in our experiments) into 1-dodecene. The above interpretation is further supported by a comparison of diamond and amorphous carbon. While diamond has a much wider bandgap than amorphous carbon (5.5 vs
