C.A. Papageorgopoulos

Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS2 has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS2. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS2 surface depend strongly on substrate temperature. Cesium is deposited on MoS2 in two adsorption states. Although MoS2 is extremely inert to O2 adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS2.

Cs deposition on layered 2H TaSe2 (0 0 0 1) surfaces: Adsorption or intercalation?

Abstract Cs was adsorbed at room temperature (RT) and at low temperature (140 K, LT) onto cleaved 2 H TaSe 2 van der Waals surfaces. At LT a Cs overlayer is formed. Cs is intercalated at RT after an initial adsorption stage. The intercalation results in changes of the Ta 4ƒ asymetric lineshapes and in a broadening of the Ta 5 d z 2 emission at the Fermi edge which is interpreted in terms of the rigid band model of intercalation.

Photoelectron spectroscopy of UHV in situ intercalated Li/TiSe2: Experimental proof of the rigid band model

Abstract Li was adsorbed at room temperature onto UHV cleaved TiSe 2 (0001) van der Waals surfaces and investigated by soft X-ray photoelectron spectroscopy. The adsorbed Li is readily intercalated into the substrate without interface decomposition. The electronic structure of the intercalated phase is analyzed from the valence band spectra giving clear evidence that the intercalation follows the rigid band model. Shifts in the Fermi level and related changes in the work function are analyzed in terms of the electronic contribution of the free energy of intercalation.

The behavior of Cs on MoS2

Abstract A combination of LEED, AES, TDS and WF measurements was used to study the adsorption of Cs on the basal plane of MoS 2 between 140 and 330 K. The results showed that at low coverage ( θ θ > 0.1 is in contrast to the uniform adsorption exhibited by Cs on metal and other semiconductors.

The effect of Cs on the oxidation of Si(111) surfaces

Abstract In this paper we report a systematic study of the effect of Cs on the oxidation of Si(111) with 7 × 7 and 1 × 1 surface structures in an UHV system by LEED, AES, TDS and work function (WF) measurements. The presence of Cs on Si(111) surfaces causes an increase of the initial sticking coefficient of oxygen by about 10 times. Near saturation Cs coverage, oxygen is initially adsorbed under the Cs layer and subsequently resides on top of it. When oxygen is adsorbed on Cs-covered Si(111) surfaces at RT and the system is subsequently heated to about 700 ° C, the Cs is removed from the surface and formation of SiO2 takes place with an equivalent thickness of about 8 A. By repeating this cycle several times the thickness of SiO2 increases. At Ts ⩾ 800 ° C, the SiO2 dissociates and desorbs in the form of SiO. The effect of Cs on the adsorption of O2 and SiO2 formation is independent of the structure of the Si(111) surface.

Adsorption of Cs and its effects on the oxidation of the Ar+ sputtered Si(100)2×1 substrate

The Ar+ sputtering causes an increase of the sticking coefficient and a maximum amount of Cs that can be deposited on the Si(100)2 × 1 surface. This is attributed to the creation of extra binding sites. On the basis of these results, it has been estimated, at RT, that the initial sticking coefficient of Cs on a flat Si(100)2 × 1 surface is about 0.5. The increased amount of Cs on the Ar+ sputtered Si(100)2 × 1 surface increases, in turn, the maximum density of subsequently adsorbed oxygen. After heating of the O/Cs/Ar+ sputtered Si surface system to ~ 750° C, the Cs is completely removed from the surface and the oxygen forms oxide with the predominant form of SiO2. For saturation coverage of Cs and the same exposure of O2 at RT, the average thickness of the SiO2 on the Ar+ sputtered Si(100)2 × 1 surface is greater than those on flat and stepped Si(100)2 × 1 surfaces. The oxide thicknesses on the above three Si(100)2 × 1 surfaces are proportional to the corresponding maximum coverages of predeposited Cs.

The influence of steps on the adsorption of Cs on Si(100)

The adsorption of Cs on Si(100)-2 × 1 was studied with LEED, AES, EELS, TDS and WF measurements under the same experimental conditions on both a flat and a 5° vicinal surface exhibiting steps in the [110] direction. The influence of the steps was investigated by comparing the experimental data from the flat and the stepped surface upon Cs adsorption at room temperature. It was found that initially Cs adsorbs preferentially on step sites with a higher sticking coefficient, larger binding energy and lower dipole moment than on the flat surface. Upon further exposure, Cs continues to adsorb on the (100)-2 × 1 terraces with a higher sticking coefficients than on the flat surface and up to the same local saturation coverage of ∼ 5.3 × 1014atomscm2. The results demonstrate the higher reactivity of steps for Cs chemisorption on Si(100)-2 × 1.

Adsorption studies of Ni on MoS2 and O2 on Ni-covered MoS2

Abstract Deposition of Ni on the basal plane of MoS2 and the interaction of this system with subsequently adsorbed oxygen have been studied in an UHV system with LEED, AES, EELS and WF measurements. For substrate temperatures at or below room temperature the deposited Ni forms small islands, which change to 3D particles on heating. At elevated substrate temperature (450 K), Ni grows to 3D particles from the early stages of its deposition. The Ni adatoms do not interact with the surface S atoms of MoS2 as Fe does. The Ni particles thus remain clean on MoS2, which is promising in heterogeneous catalysis. When the MoS2Ni system is exposed to oxygen the latter is adsorbed only on Ni. The interaction of the Ni adsorbate with oxygen is quite similar to that of oxygen with metallic Ni substrates.

Adsorption studies on Ar+ -sputtered MoS2(0001)

Abstract In this work we report a study of Ar + -sputtered MoS 2 (0001) before and after its interaction with gas and metal adsorbates. The study took place in an UHV system with LEED, AES, EELS and WF measurements. The adsorbates were O 2 and Fe. Argon-ion bombardment of MoS 2 (0001) caused a displacement of S atoms in the top layer thus exposing areas of the Mo underlayer on the surface. The interaction of deposited oxygen with the Mo areas exposed on the surface of the Ar + -sputtered MoS 2 (0001) was quite similar to that of O 2 with Mo metal substrate. The Fe adsorbate was deposited on Ar + -sputtered MoS 2 (0001) in two different ways. It formed uniform layers on the Mo areas while on the rest area covered by S, it formed small particles.

Ba deposition on Si (1 0 0)2 × 1

The Ba deposition on Si(1 0 0)2 × 1 at room temperature has been investigated by LEED, AES, TDS, EELS and work function measurements. Ba grows, though disordered, layer by layer with a constant sticking coefficient. Barium on Si(1 0 0)2 × 1 gives rise to two adsorption states which exhibit relatively high binding energy for Θ ≤ 2 ML, a strong Ba-Si ionic interaction and relatively low binding energy for Θ ≤ 2 ML where the Ba overlayer has a metallic character. Heating of the Ba-Si interface at T < 700°C promotes BaSi interaction, probably with a tendency to form silicide compounds. Higher temperatures cause the appearance of the 2 × 4, 2 × 1 and 2 × 3 diffraction patterns.