Mn/GaAs(100)界面的形成和化学反应的研究

利用低能电子衍射(LEED)、X射线光电子能谱(XPS)、电子能量损失谱(EELS)、紫外光电子能谱(UPS),对室温下Mn在GaAs(100)4×1表面的淀积过程进行了研究。研究结果表明,当锰的覆盖度θ≥0.25nm时,LEED图案完全消失,表明Mn没有生长成单晶。LEED,EELS的结果都表明淀积初期是层状生长的。对XPS的Ga2p_3/2,As2p_3/2的峰形、强度进行分析,可以知道在很小的覆盖度下,Mn就与衬底反应。置换出的Ga被局限在离原来的界面约3nm 关键词：     

Interaction of CO with surface PdZn alloys

The adsorption and bonding configuration of CO on clean and Zn-covered Pd(1 1 1) surfaces was studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). LEED and TPD results indicate that annealing at 550 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(1 1 1) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/Pd(1 1 1) alloy surfaces compared to clean Pd(1 1 1). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al₂O₃ catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.     

Spectroscopic identification of the reaction intermediates in the thermal decomposition of trimethylindium at GaAs(100) surfaces

High resolution electron energy loss spectroscopy (HREELS) has been used to study the adsorption and thermal decomposition of trimethylindium (TMIn) on Ga-terminated GaAs(100) surfaces. HREEL spectra recorded for adsorption at room temperature are dominated by strong CH₃ deformation and stretching modes and indicate that the surface species is based on methyl groups. The intensities of these bands decrease with increasing temperature consistent with a primary decomposition route involving the loss of CH₃ groups from the surface. A small upward shift in the frequency of the symmetric and asymmetric CH₃ deformation modes is also observed with increasing temperature and indicates that decomposition takes place via an exchange reaction in which CH₃ groups switch from In to Ga due to the stronger Ga-C bond. At temperatures greater than 350°C, the spectra are dominated by CH₂ rocking, deformation and stretching vibrations. The presence of a surface methylene species at elevated temperatures suggests a second, minority decomposition pathway which involves dehydrogenation of surface CH₃ groups to CH₂.     

Comparison of site-specific densities of states of Ga and As in cleaved and sputtered GaAs(110) by means of Auger line shapes

Site-specific densities of states for the Ga and As sites in GaAs(110) are derived from the M₁M_4,5V Auger lines and compared for the cleaved and sputtered surfaces. X-ray photoelectron spectroscopy, scanning electron microscopy, and LEED results for these surfaces are also presented. The results are interpreted in terms of a structure for the sputtered surface that consists of a two-phase mixture of Ga and disordered GaAs.     

Electron beam-adsorbate interactions on silicon surfaces

The interactions which occur between electron beams in the energy range 0.5–2.5 keV, with currents of 0.1–1.0 microA and various adsorbates (H₂, CO, CH₄ and C₂H₄) on silicon surfaces have been investigated. The accumulation of beam induced dissociation products on the surface has been monitored by Auger spectroscopy, and the extent of electron stimulated desorption of neutral molecules has been determined mass spectroscopically. Thermal desorption spectra for various gases have also been obtained in order to compare adsorption behaviour with and without the presence of an electron beam. It is concluded that serious experimental errors may occur when LEED and AES are used in adsorption studies, particularly where comparatively weak binding energies are involved.     

Adsorption of glycine on a NiAl(1 1 0) alloy surface

The adsorption and desorption of glycine (NH₂CH₂COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δφ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH₂CH₂COO⁻). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al-O layer.     

LEED-Auger characterization of GaAs during activation to negative electron affinity by the adsorption of Cs and O

A combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been used to study the formation of the negative electron affinity (NEA) condition on surfaces of p-type, degenerate, (100) and (111) GaAs. Activation to NEA is achieved by adsorbing Cs and O onto atomically clean GaAs in repetitive cycles of first Cs and then O. Before activation, the clean GaAs surfaces exhibit their characteristic LEED patterns. However, once obtained, there is no significant correlation between the quality of these LEED patterns and the final activation. The adsorption of both Cs and O during activation to NEA is amorphous. Auger measurements have shown that the first photoemission maximum occurs after the adsorption of about a half monolayer of Cs. The initial O adsorption occurs on the GaAs surface between the Cs atoms. The adsorbed O interacts strongly with Cs at any stage during the activation. Peak photosensitivities, after completion of the Cs and O adsorptions, were in the range 400 to 1100 $\text{μA}\text{lumen}$. The final activation does not correlate with the quantity of Cs and O on the surface. The temperature dependence of the photosensitivity of NEA GaAs (100) activated at ?170°C has a broad maximum at about ?50°C and a subsidiary maximum at about 160°C. In addition, the photoemission at ?170°C can be either increased or decreased by having heated the sample up to 200°C, even though no Cs or O desorption has taken place. These results can be traced to changes in work function rather than to changes in bulk properties. While the LEED patterns from clean GaAs show no structural changes with temperature, such changes are observed when Cs is on the surface. It is suggested that changes both in photoemission and in LEED patterns are due to the temperature-induced mobility of Cs on GaAs. An atomic model for the NEA surface is discussed in terms of a layer of Cs and O atoms about 10 Å thick on the GaAs.     

Structure and vibrations of CO adsorbed on the Ni(100)p(2 × 2)O and c(2 × 2)O surfaces

Chemisorption of CO on the Ni(100)p(2 × 2)O and c(2 × 2)O surfaces has been investigated by high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). At 175 K CO adsorption on Ni(100)p(2 × 2)O saturates at about 1 L exposure in a structure interpreted to be Ni(100)p(2 × 2)O—p(2 × 2)CO. The CO layer is stable at 175 K but desorbs readily around 300 K. The EEL spectrum for p(2 × 2)CO shows vibrational losses at 46 meV and 245 meV interpreted to be due to excitations of the Ni-C and C-O stretching vibrations of CO molecules bridge bonded to two nearest neighbour Ni atoms. This interpretation is also supported by the LEED observations. For the preceeding dilute CO layer the vibrational loss spectrum reveals CO adsorption both to Ni bridge sites and hollow sites. At 175 K CO does only adsorb stationary on p(2 × 2)O defects in the Ni(100)c(2 × 2)O surface and not at all on epitaxially grown NiO(111) and (100) surfaces.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Adsorption states and Ga depletion during oxygen adsorption on clean polar GaAs surfaces

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1&#x0304;1&#x0304;1&#x0304;)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O₂. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga₂O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form.     

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd_0.75Cu_0.25(111) single crystal surface with surface composition of about Pd_0.7Cu_0.3. The main objective was to make a comparison with the previously studied Pd_0.67Ag_0.33(111) (surface composition Pd_0.1Ag_0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10⁻⁹ mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

Effects of adsorbed carbon and oxygen on the chemisorption of H2 and Co on Mo(100)

The deposit of carbon and oxygen adatoms on Mo(100) was characterized by AES and LEED. Carbon was introduced by the thermal cracking of ethylene; several ordered structures were observed as a function of coverage with carbon atoms residing on four-fold sites. The Mo(100)—O system exhibited two different sequences of LEED patterns depending on the adsorption temperature of oxygen. The effects of adsorbed carbon and oxygen on the chemisorption properties of Mo(100) was investigated by FDS. The presence of either carbon or oxygen severely hindered the ability of Mo(100) to dissociatively adsorb hydrogen or carbon monoxide. The amount of CO dissociated was directly related to the available four-fold sites on the carbide surfaces. The molecular adsorption of CO was not significantly affected by the adlayers. It was found that one monolayer of adsorbed oxygen reduced the binding energy of molecular CO considerably more than the same amount of adsorbed carbon. A continuous shift in the binding energy of CO with the C/O ratio on the surface was observed.     

Angular-resolved photoemission from GaAs(110) surfaces with adsorbed Al

GaAs(110) surfaces with adsorbed Al were studied by a combination of angular-resolved valence band photoemission, Ga 3d core level photoemission, low energy electron loss spectroscopy and Auger electron spectroscopy. At room temperature Al is adsorbed on top of GaAs. After heat treatment the compound AlAs is formed at the surface, which is used as a substrate for Ga adsorption to form the inverted structure of the system GaAs + Al.     

Els and leed study of CO adsorption on Pd(001) and Pd(001) 6° [110]

LEED studies indicate that both CO and carbon adsorb differently on Pd(001) surfaces with and without steps. However, electron energy loss spectra for Pd(001) samples with and without ordered steps show no detectible surface structural sensitivity to CO adsorption. For both samples an additional loss feature at 13.5 eV appears upon CO adsorption. This feature is identified as a (1π?5σ) to 2π^★ type intramolecular electronic excitation, and it appears to be a universal feature of molecular CO adsorption on metals. A second loss expected in the 6–7 eV range associated with a d to 2π^★ type charge-transfer excitation was not detected. Either the transition probability is low for this excitation or hybridization with surface plasmons may obscure its identification.     

用HREELS研究氢在GaAs和InP的(111),(111)表面上的吸附

氢在GaAs和InP表面上的吸附可以用高分辨率电子能量损失谱(HREELS)来探测。Ga—H,As—H,In—H和P—H键的伸缩振动各自对应于不同的能量损失。但是As—H振动极容易和Ga—H振动追加声子损失相混淆,只有从损失峰的相对强度比较上来区别。实验得到吸附的氢与表面原子的成键情况取决于表面的原子结构及电子分布。对于GaAs(111)面,低暴露量时只形成Ga—H键,而高暴露量时还可以形成As—H键。而InP(111)表面由于是经过磷气氛退火处理的,在低暴露量下In—H与P—H键均可形成。InP(Ⅲ)面上只看到P—H损失峰,说明这个表面是完全以P原子结尾的。在(Ⅲ)面上出现小面的情形,则表面Ⅲ族和Ⅴ族原子均可同氢成键。 关键词：     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

Adsorption and thermal reaction of short-chain alkanethiols on GaAs(1 0 0)

By means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation, we investigated the adsorption and thermal decomposition of alkanethiols (RSH, R = CH₃, C₂H₅, and C₄H₉) on a GaAs(1 0 0) surface. All chemisorbed alkanethiols can deprotonate to form thiolates below 300 K via dissociation of the sulfhydryl hydrogen (-SH). Two types of thiolates species are observed on GaAs(1 0 0), according to adsorption on surface Ga and As sites. The thiolates adsorbed on a Ga site preferentially recombine with surface hydrogen to desorb as a molecular thiol at 350-385 K. The thiolate on the As site exhibits greater thermal stability and undergoes mainly dissociation of the C-S bond at ∼520 K, independent of the alkyl chain length. The decomposition of CH₃S either directly desorbs CH₃ or transfers the CH₃ moiety onto the surface. The surface CH₃ further evolves directly from the surface at 665 K. The dissociations of C₂H₅S and C₄H₉S yield surface C₂H₅ and C₄H₉, which further decompose to desorb C₂H₄ and C₄H₈, respectively, via β-hydride elimination. The complete decomposition of alkanethiol leads to the formation of surface S without deposition of carbon. Adsorption of CH₃SSCH₃ results in the formation of surface CH₃S at initial exposures via scission of the S−S bond. Compared with the adsorption of CH₃SH, the CH₃S on the Ga site exhibits greater thermal stability because surface hydrogen is absent. At a high exposure, CH₃SSCH₃ can absorb molecularly on the surface and decompose to desorb CH₃SCH₃ via formation of a CH₃SS intermediate.     

用HREELS研究氢在GaAs和InP的（111），（III）表面上的吸附

氢在GaAs, 和InP 表面上的吸附可以用高分辨率电子能量损失谱(HREELS) 来探测.Ga-H侧, As-H , In-H 和P-H 键的伸缩振动各自对应于不同的能量损失. 但是A-H 振动极容易和Ga-H 振动追加声子损失相混淆, 只有从损失峰的相对强度比较上来区别。实验得到吸附的氢与表面原子的成键情况取决于表面的原子结构及电子分布. 对于GaAs (1 1 1)面, 低暴露量时只形成Ga-H 键, 而高暴露量时还可以形成As-H 键.而InP(1 1 1 ) 表面由于是经过磷气氛退火处理的, 在低暴露量下In 一H 与P一H 键均可形成. InP (III) 面上只看到P-H 损失峰, 说明这个表面是完全以P 原子结尾的. 在(1 1 1 ) 面上出现小面的情形, 则表面III族和V族原子均可同氢成键. 关键词：     

Room-temperature-adsorption behavior of acetic anhydride on a TiO2(1 1 0) surface

We have studied the adsorption structure of acetic anhydride on a TiO₂(1 1 0) surface using XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction) and HREELS (high resolution electron energy loss spectroscopy) to determine the origins of the unique adsorption properties of carboxylic acids on a TiO₂(1 1 0) surface. The C 1s XPS data indicated that the saturation carbon amount of adsorbed acetic anhydride was 12 ± 3% larger than that of the adsorbed acetic acid. LEED showed p(2 × 1) weak spots for the acetic anhydride adsorbed surface. The HREELS spectra revealed the dissociative adsorption of acetic anhydride. Based on these findings, we concluded that the neutralization of the bridging oxygen atoms associated with the dissociative adsorption is necessary for the stable adsorption of carboxylates on the 5-fold Ti sites.