X-ray photoelectron spectroscopic analysis of Si nanoclusters in SiO2 matrix

We investigated silicon nanoclusters Si(nc) in a SiO2 matrix prepared by the plasma-enhanced chemical vapor deposition technique, using X-ray photoelectron spectroscopy (XPS) with external voltage stimuli in both static and pulsed modes. This method enables us to induce an additional charging shift of 0.8 eV between the Si2p peaks of the oxide and the underlying silicon, both in static and time-resolved modes, for a silicon sample containing a 6 nm oxide layer. In the case of the sample containing silicon nanoclusters, both Si2p peaks of Si(nc) and host SiO2 undergo a charging shift that is 1 order of magnitude larger (>15 eV), with no measurable difference between them (i.e., no differential charging between the silicon nanoclusters and the oxide matrix could be detected). By use of a measured Auger parameter, we estimate the relaxation energy of the Si(nc) in the SiO2 matrix as -0.4 eV, which yields a -0.6 eV shift in the binding energy of the Si(nc) with respect to that of bulk Si in the opposite direction of the expected quantum size effect. This must be related to the residual differential charging between the silicon nanoclusters and the oxide host. Therefore, differential charging is still the biggest obstacle for extracting size-dependent binding energy shifts with XPS when one uses the oxide peak as the reference.     

High-resolution soft X-ray photoelectron spectroscopic studies and scanning auger microscopy studies of the air oxidation of alkylated silicon(111) surfaces

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si(+), Si(2+), Si(3+), and Si(4+) were detected on the Cl-terminated surface, with the highest oxidation state (Si(4+)) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p(3/2) peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monoleyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH(3)-, C(2)H(5)-, or C(6)H(5)CH(2)-terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed approximately 0.20 ML of oxide. This oxide was principally composed of Si(+) and Si(3+) species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p(3/2) peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7 degrees vs < or =0.5 degrees off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.     

High-resolution X-ray photoelectron spectroscopic studies of alkylated silicon(111) surfaces

Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.     

Determination of the interface band alignment of Mg2Si/4H-SiC heterojuction for potential photodetector application

The Mg₂Si/4H-SiC heterojunction was prepared by radio frequency (RF) magnetron sputtering technique. The binding energies of Mg 2p, Si 2p, and C 1s core levels and the maxima of valence band were measured by X-ray photoelectron spectroscopy (XPS). Using the optical bandgap of Mg₂Si (0.78 eV) and 4H-SiC (3.25 eV), the band offsets of valence band (VBO) and conduction band (CBO) at Mg₂Si/4H-SiC interface were identified as 1.47 and 1.00 eV, respectively. The band alignment was evaluated to be type-I band alignment. The Mg₂Si/4H-SiC heterojunction could be a promising candidate for the infrared (IR) photodetector.     

Depth profiling of charging effect of Si nanocrystals embedded in SiO2: a study of charge diffusion among Si nanocrystals

Si nanocrystal (nc-Si) embedded in SiO2 thin film is synthesized with low-energy Si ion implantation. Depth profiling of the charging effect of the nc-Si is determined from X-ray photoemission measurement. It is observed that there is a strong correlation between the depth profile of the charging effect and the nc-Si depth distribution. The charging effect is found to decrease with the increase of nc-Si concentration and to vanish when a densely stacked nanocrystal layer is formed. The phenomenon is attributed to the charge diffusion among the nanocrystals. The charge diffusion in the nanocrystal layer may have an important implication for nanocrystal flash memory. When such a layer is used as the charge-storage layer in the memory cells, the stored charges could be lost due to the rapid charge diffusion among the nc-Si if a single defect exists in the tunneling oxide, causing a reliability problem in data retention.     

Scanning photoemission spectromicroscopic study of 4-nm ultrathin SiO(3.4) protrusions probe-induced on the native SiO(2) layer

Atomic force microscopy probe-induced large-area ultrathin SiO(x) (x ≡ O/Si content ratio and x > 2) protrusions only a few nanometers high on a SiO(2) layer were characterized by scanning photoemission microscopy (SPEM) and X-ray photoemission spectroscopy (XPS). SPEM images of the large-area ultrathin SiO(x) protrusions directly showed the surface chemical distribution and chemical state specifications. The peak intensity ratios of the XPS spectra of the large-area ultrathin SiO(x) protrusions provided the elemental quantification of the Si 2p core levels and Si oxidation states (such as the Si(4+), Si(3+), Si(2+), and Si(1+) species). The O/Si content ratio (x) was evidently determined by the height of the large-area ultrathin SiO(x) protrusions.     

硅晶片上超薄氧化硅层厚度纳米尺寸效应的XPS研究

用X射线光电子能谱(XPS)测定了一系列厚度经过国际比对准确已知的硅晶片上的超薄(1.45nmd7.2nm)氧化硅膜的Si2p电子能谱和价带谱.结果表明:SiO2膜厚d2nm时,Si2p结合能最低,其原因可归结于此时光电离空穴既有来自SiO2中的原子极化对空穴的原子外弛豫,也有来自衬底Si的电荷移动对空穴的屏蔽(有效屏蔽距离大约是(2.5±0.6)nm);当d3nm时Si2p结合能增大,此时只有来自SiO2的原子外弛豫,d较小者的Si2p结合能较高.SiO2的价带电子结构也与其厚度纳米尺寸效应有关:当d2nm时价带中SiO2的O2p非成键电子峰的相对强度较强,O2p—Si3p和O2p—Si3s成键电子峰较弱.     

Grazing incidence X-ray photoemission spectroscopy of SiO2 on Si

We have applied grazing incidence X-ray photoemission spectroscopy to the determination of the thickness of SiO₂ layers on Si, as well as surface carbon that is present. The measurements take advantage of the different optical constants of the layers. X-rays incident on the surface at grazing angle undergo total external reflection, where the fields in each layer are subject to highly non-linear changes as a function of incidence angle. X-ray photoemission excited by these fields gives information on atomic species, chemical state, and layer thickness. Simultaneous fits are made to the photoemission spectra in each layer. The method is illustrated for a thermally grown oxide layer and a native oxide on Si.     

In situ x-ray photoelectron spectroscopic and density-functional studies of Si atoms adsorbed on a C60 film

The interaction between C(60) and Si atoms has been investigated for Si atoms adsorbed on a C(60) film using in situ x-ray photoelectron spectroscopy (XPS) and density-functional (DFT) calculations. Analysis of the Si 2p core peak identified three kinds of Si atoms adsorbed on the film: silicon suboxides (SiO(x)), bulk Si crystal, and silicon atoms bound to C(60). Based on the atomic percent ratio of silicon to carbon, we estimated that there was approximately one Si atom bound to each C(60) molecule. The Si 2p peak due to the Si-C(60) interaction demonstrated that a charge transfer from the Si atom to the C(60) molecule takes place at room temperature, which is much lower than the temperature of 670 K at which the charge transfer was observed for C(60) adsorbed on Si(001) and (111) clean surfaces [Sakamoto et al., Phys. Rev. B 60, 2579 (1999)]. The number of electrons transferred between the C(60) molecule and Si atom was estimated to be 0.59 based on XPS results, which is in good agreement with the DFT result of 0.63 for a C(60)Si with C(2v) symmetry used as a model cluster. Furthermore, the shift in binding energy of both the Si 2p and C 1s core peaks before and after Si-atom deposition was experimentally obtained to be +2.0 and -0.4 eV, respectively. The C(60)Si model cluster provides the shift of +2.13 eV for the Si 2p core peak and of -0.28 eV for the C 1s core peak, which are well corresponding to those experimental results. The covalency of the Si-C(60) interaction was also discussed in terms of Mulliken overlap population between them.     

Photoemission from sodium on ice: a mechanism for positive and negative charge coexistence in the mesosphere

Photoemission from sodium deposited on ice films is described. Deposition of 0.02 ML of sodium is found to dramatically reduce the threshold for photoemission from the ice film to (2.3+/-0.2) eV. Thus, the cross-section for photoemission reaches >10(-18) cm2 in the visible region of the spectrum. It is proposed that the initial state is a solvated electron on the ice surface, which is supported by optical transmission spectroscopy. The potential significance of these results in understanding unexplained charging phenomena in the mesosphere is discussed.     

Charge referencing issues in XPS of insulators as evidenced in the case of Al‐Si‐N thin films

Accurate charge referencing in XPS of insulating specimens is a delicate issue. This difficulty is illustrated in the case of Al‐Si‐N composite thin films deposited by reactive magnetron sputtering with variable composition from pure aluminum nitride to pure silicon nitride. The samples were mounted with Au‐coated metallic clamps. Argon sputter cleaning was required to remove a surface native oxide before analysis. For charge referencing implanted argon atoms from the sputter gas and a small amount of gold re‐deposited from the metallic clamps onto the specimen surface during sputter cleaning were evaluated. For the argon atoms, a surprisingly large chemical shift (～1 eV) and a significant peak broadening (0.6 eV) of the Ar 2p_3/2 photoelectron line were found with varying the Si content of the films. This could be related to chemical and structural changes of the Al‐Si‐N films. Hence implanted argon could not be used for charge referencing of Al‐Si‐N samples. In contrast to the implanted argon, the Au 4f_7/2 line width of the gold re‐deposited onto the sample surface did not depend on the Si content of Al‐Si‐N films. A constant energy shift (～1.2 eV) of the Au 4f_7/2 line as compared with bulk gold was, however, found, which was related to the size of gold particles formed on the insulating films. Therefore gold could be reliably used to study chemical shifts of sample‐relevant species in Al‐Si‐N films, but the absolute binding energies of Al 2p, Si 2p and N 1s photoelectrons could not be determined. Copyright © 2011 John Wiley & Sons, Ltd.     

Bonding structure of phenylacetylene on hydrogen-terminated Si(111) and Si(100): surface photoelectron spectroscopy analysis and ab initio calculations

Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s → π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH(2)) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.     

FT-IR、XPS和DFT研究水杨酸钠在针铁矿或赤铁矿上的吸附机理

采用傅里叶变换红外(FT-IR)光谱、X射线光电子能谱(XPS)以及基于周期平面波的密度泛函理论(DFT)分别研究了水杨酸钠在针铁矿或赤铁矿表面上的吸附结构,并将计算得到的光电子能谱移动(CLS)和电荷转移与实验得到的XPS结果进行对比。FT-IR结果表明,水杨酸钠可能以双齿双核(V)和双齿单核(IV)的形式分别吸附于针铁矿或赤铁矿表面。由DFT计算结果可知,水杨酸钠在针铁矿(101)晶面上形成双齿双核化合物(V)的吸附能为-5.46 eV。而水杨酸钠在针铁矿(101)晶面上形成双齿单核化合物(IV)的吸附能为3.80 eV,因此水杨酸钠在针铁矿上基本不以双齿单核化合物(IV)构型存在。水杨酸钠在赤铁矿(001)晶面上形成双齿单核化合物(IV)时吸附能为-4.07 eV,说明水杨酸钠在赤铁矿(001)晶面上形成了双齿单核化合物(IV)。另外,理论计算的针铁矿(101)晶面上吸附位点铁原子的Fe 2p的CLS值(-0.68 eV)与实验观察到的Fe 2p的CLS值(-0.5 eV)吻合。理论计算的赤铁矿(001)晶面上吸附位点铁原子的Fe 2p的CLS值(-0.80 eV)与实验观察到的Fe 2p的CLS值(-0.8 eV)吻合。因此,水杨酸钠吸附在针铁矿表面时能够通过羧酸基团上一个氧原子和酚羟基上的氧原子与针铁矿(101)表面上的两个铁原子形成双齿双核(V)结构,而在赤铁矿(001)表面上,水杨酸钠中羧酸基团上一个氧原子和酚羟基上的氧原子与赤铁矿(001)表面上的一个铁原子形成了双齿单核(IV)结构。     

X-ray-induced production of gold nanoparticles on a SiO(2)/Si system and in a poly(methyl methacrylate) matrix

Prolonged exposure to X-rays of HAuCl(4) deposited from an aqueous solution onto a SiO(2)/Si substrate or into a poly(methyl methacrylate) (PMMA) matrix induces reduction of the Au(3+) ions to Au(0) and subsequent nucleation to gold nanoclusters as recorded by X-ray photoelectron spectroscopy. The corresponding major oxidation product is determined as chlorine {HAuCl(4)(ads) + X-rays --> Au(ads) + (3/2)Cl(2)(ads) + HCl(ads)}, which is initially adsorbed onto the surface but eventually diffuses out of the system into the vacuum. The reduced gold atoms aggregate (three-dimensionally) into gold nanoclusters as evidenced by the variation in the binding energy during X-ray exposure, which starts as 1.3 eV but approaches a value that is 0.5 eV higher than that of the bulk gold. The disappearance of the oxidation product (Cl2p signal) and the growth of the nanoclusters (related to the measured binding energy difference between the Si2p of the oxide and Au4f of the reduced gold) exhibit first-order kinetics which is approximately 3 times slower than the reduction of Au(3+), indicating that both of the former processes are diffusion controlled. Similarly, gold ions incorporated into PMMA can also be reduced and aggregated to gold nanoclusters using 254 nm deep UV irradiation in air evidenced by UV-vis-NIR absorption spectrocopy.     

Circular dichroism in the angle-resolved C 1s photoemission spectra of gas-phase carvone enantiomers

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8+/-0.2 and 289.8+/-0.2 eV, respectively. The remaining C-H C 1s levels substantially overlap under an intense central peak centered at 290.5+/-0.2 eV. The angle-resolved photoemission from the carbonyl carbon C=O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of approximately 6% at the magic angle of 54.7 degrees to the light beam propagation direction. This corresponds to an expected 0 degrees -180 degrees forward-backward electron emission asymmetry of approximately 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.     

XPS spectra of chromium monosilicides and disilicides obtained by in situ fractured clean surfaces

X-ray photoelectron spectroscopy spectra of chromium monosilicide (CrSi) and disilicide (CrSi₂) were collected from a clean surface prepared by fracturing the bulk silicide compound in a spectrometer under ultrahigh vacuum; the analytical procedure for the phase identification of the Cr–Si system was examined. A negligible binding energy shift was observed in the Cr 2p_3/2 level between elemental Cr and CrSi₂, whereas the energy of CrSi was 0.2 eV lower than that of Cr and CrSi₂. The satellite peak in the Cr 2p spectra originating from the plasmon-loss phenomena was found only for CrSi and CrSi₂. The binding energy of Si 2p shifted, reflecting the silicide phases; the energy of CrSi₂ and CrSi was 0.3 eV higher and lower, respectively, than that of elemental Si. Although a slight difference in the spectral shape was observed in the valence band region, the phase identification was considered unreliable. However, the energy shifts of Si 2p and the presence of the plasmon-loss peak in the Cr 2p spectra provide important insights into the phase identification of the Cr–Si system.     

Influence of submonolayer sodium adsorption on the photoemission of the Cu(111)/water ice surface

Photoemission from an ice film deposited on Cu(111) as a function of thickness has been observed in the presence and absence of sodium atoms at the surface-vacuum interface. For either adsorbate alone and photon energies below 4 eV, two-photon photoemission from the Cu(111) substrate dominates. The Cu(111) photoelectron spectrum is perturbed by low coverages of Na, and its intensity is strongly attenuated by a few monolayers of ice. For a low density amorphous ice film, strong charging effects are observed. For ice films annealed to yield either the dense amorphous or crystalline phase, this effect is absent. Deposition of only 0.02 monolayer of Na leads to a dramatic decrease in the threshold for photoemission to 2.3+/-0.2 eV. Thus, photoelectrons are generated by visible radiation in a one-photon process with a cross section that exceeds 10(-18) cm(2). The initial state for the photoemission is identified as a metastable surface trapped electron, which decays thermally with an activation energy of 10+/-2 kJ mol(-1). Quantum calculations are described which support this model and show that the Na atom is accommodated in the first layer of the ice surface.     

Excitation energy dependence for the Li 1s X-ray photoelectron spectra of LiMn2O4.

The Li 1s XPS (X-ray Photoelectron Spectroscopy) spectra of LiMn2O4, which is one of the major positive-electrode materials in lithium-ion rechargeable batteries, and MnO2 as a reference material, were measured by a laboratory-type XPS spectrometer. The Li 1s peak was not observed in the spectra excited by the Mg Kalpha line (1253.6 eV), because the Li 1s peak overlapped the background of the Mn 3p peak of LiMn2O4. The photoionization cross section of Mn 3p was larger than that of Li 1s for Mg Kalpha excitation. Therefore, the XPS measurement of LiMn2O4 by soft X-ray synchrotron excitation was carried out at beamline BL-7B on NewSUBARU synchrotron facility. Excitation energies of 110, 120, 130, 140, 150 and 151.4 eV were selected. The Li 1s peak was clearly observed in these XPS spectra. In order to investigate the excitation energy dependence, the area ratio of the Li 1s and Mn 3p peaks in the XPS spectra was plotted against the excitation energy. As a result, when the excitation energy was 110 eV, the area ratio had the maximum value.     

Structure-dependent optical properties of single-walled silicon nanotubes

The electron excitations of Single-Walled Silicon Nanotubes (SWSiNTs), with sp(2) and sp(3) hybridization, were studied using the localized-density-matrix (LDM) method with INDO/S parameters. Strong anisotropic characteristics of the dynamic polarizabilities were found for all the nanotubes. The transitional intensity along the tubular axis is much larger than that perpendicular to the axis for all the nanotubes. The optical gaps of sp(3)-hybridized infinitely-long pentagonal SWSiNTs are near 3.0 eV and 4.7 eV owing to σ-σ* transitions along the direction of the tubular axis. The optical gaps of sp(2)-hybridized infinitely-long armchair SWSiNTs along the tube axis direction are about 0.7 eV and 2.4 eV for Si(3,3) SWSiNTs and 0.7 eV and 2.7 eV for Si(4,4) SWSiNTs. The former peak at 0.7 eV originated from π-π* electron transitions and the latter peak at 2.4 eV or 2.7 eV originated from σ-σ* electron transitions. Meanwhile, the intensities of π-π* electron transitions are stronger than those of σ-σ* electron transitions in SWSiNTs. The low sp(2) transition energy derived from the weak overlap of unpaired p(z) orbitals of silicon atoms. Moreover, the electronic excitations of zigzag SWSiNTs are similar to those of armchair structures. This indicates that sp(2)-hybridized silicon nanotubes possess much greater potential for application in optical fields.