Orientation-dependent ionization potential of CuPc and energy level alignment at C60/CuPc interface

The electronic structure evolution of interfaces of fullerene (C₆₀) with copper phthalocyanine (CuPc) on highly oriented pyrolytic graphite (HOPG) and on native silicon oxide has been investigated with ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The LUMO edge of C₆₀ was found to be pinned at the interface with CuPc on SiO₂. A substantial difference in the electron affinity of CuPc on the two substrates was observed as the orientation of CuPc is lying flat on HOPG and standing up on SiO₂. The ionization potential and electron affinity of C₆₀ were not affected by the orientation of CuPc due to the spherical symmetry of C₆₀ molecules. We observed band bending in C₆₀ on the standing-up orientation of CuPc molecules, while the energy levels of C₆₀ on the flat-lying orientation of CuPc molecules were observed to be flat. The observation points to a dependence of photoexcited charge transfer on the relative molecular orientation at the interface.     

Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.     

Probing the interaction at the C60-SiC nanomesh interface

Synchrotron-based high-resolution photoemission spectroscopy (PES) and in situ scanning tunneling microscopy (STM) are used to investigate the interaction at the C₆₀-SiC nanomesh interface during the sequential deposition and subsequent desorption of C₆₀ molecules. A weak charge transfer occurs at the C₆₀-nanomesh interface, involving electrons transferring from nanomesh to C₆₀ overlayer. The interface interaction originated from the weak charge transfer at the C₆₀-nanomesh interface is stronger than C₆₀ intermolecular interaction (e.g., van-der-Waals force), facilitating the layer-by-layer growth for the first two layers of C₆₀ on SiC nanomesh. The highly corrugated nanomesh surface results in an anisotropic diffusion and high diffusion barrier of C₆₀ on top, and thereby leads to the formation of irregularly shaped C₆₀ islands under submonolayer condition. In contrast, C₆₀ diffusion on HOPG and Ag(1 1 1) surfaces is rather isotropic, resulting in the formation of hexagonally shaped C₆₀ islands with smooth domain boundaries. STM results show the partial desorption of C₆₀ molecules from the SiC nanomesh surface after annealing the 1 ML C₆₀ sample (complete wetting layer of C₆₀ on SiC nanomesh) at around 150 °C for 20 min. Thorough desorption of C₆₀ molecules and full recovery of the clean SiC nanomesh are observed after annealing at around 200 °C for 20 min. In situ PES and STM experiments clearly demonstrate that C₆₀ adsorption and desorption processes do not affect the underlying SiC nanomesh structure, revealing its thermal stability and chemical inertness to C₆₀ molecules.     

Self-assembly of C60 into highly ordered chain-like structures on HOPG observed at ambient conditions

The observation of chain-like structures of self-assembled C₆₀ molecules on HOPG surfaces at room temperature in aerial atmosphere by means of scanning tunneling microscopy is reported. The ca. 2.5 nm center-to-center distance between two fullerene molecules is much larger than in the close-packed layered or film structures of C₆₀ usually found on HOPG surfaces.     

Sputtering of thin benzene and polystyrene overlayers by keV Ga and C60 bombardment

The mechanisms of ion-stimulated desorption of thin organic overlayers deposited on metal substrates by mono- and polyatomic projectiles are examined using molecular dynamics (MD) computer simulations. A monolayer of polystyrene tetramers (PS4) physisorbed on Ag{1 1 1} is irradiated by 15 keV Ga and C₆₀ projectiles at normal incidence. The results are compared with the data obtained for a benzene overlayer to investigate the differences in sputtering mechanisms of weakly and strongly bound organic molecules. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C₆₀, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C₆₀ and Ga projectiles. Our data indicate that chemical analysis of the very thin organic films performed by detection of sputtered neutrals will not benefit from the use of C₆₀ projectiles.     

XPS study on the evaporation of gold submonolayers on carbon surfaces

We investigated the early nucleation stages of evaporated gold submonolayers on different carbon surfaces (pristine HOPG, argon-ion irradiated HOPG and amorphous carbon). Gold core-level and valence band spectra were measured by monochromatised X-ray photoelectron spectroscopy (MXPS). The Au 4f spectra for the lowest coverages (0.1 Å equivalent thickness) on irradiated HOPG and amorphous carbon surprisingly exhibited two well-separated doublets. We attribute this phenomenon to a bimodal particle size distribution caused by gold atom pinning at carbon defect sites. Deposition at elevated temperatures (on irradiated HOPG) opens a possibility to grow particles preferentially on defect sites. The influence of carbon surface defects on the cluster morphology was checked by SEM imaging. These results are interesting for future applications as they help to improve control over metal nanodots growth.     

The chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

Hydrogen bonds induced supramolecular self-assembly of azobenzene derivatives on the highly oriented pyrolytic graphite surface

The self-assembly of azobenzene derivatives (C_nAzCOOH) with various lengths of peripheral alkyl chains (with carbon number of n = 8, 10, 12, 14, 16) were observed by scanning tunneling microscopy on highly oriented pyrolytic graphite (HOPG) surface. The effect of van der Waals interactions and the intermolecular hydrogen bonding on the two-dimensional self-assembly was systematically studied. No alkyl-chain length effect was observed according to the STM images. All kinds of C_nAzCOOH adopting the same pattern self-assembled on the HOPG surface, suggesting the formation of the two-dimensional structures was dominated by the hydrogen bonding of the functional groups. It could be found that two C_nAzCOOH molecules formed a hydrogen-bonded dimer with “head-to-head” fashion as expected; however, the dimers organized themselves in the form of relative complex lamellae. Three dimers as a group arranged side by side and formed a well-defined stripe with periodic dislocations due to the registry mechanism of the alkyl chain with the underlying HOPG surface. The hydrogen bonds between the adjacent dimers in one lamella were formed and dominated the self-assembled pattern.     

Intramolecular structures of C60 and C84 molecules on Si(1 1 1)-7 × 7 surfaces by scanning tunneling microscopy

The intramolecular features of carbon 60 and carbon 84 molecules on Si(1 1 1)-7 × 7 surfaces were studied under a UHV-scanning tunneling microscope. Carbon molecules preferentially appear in faulted halves, rather than in unfaulted halves and corner holes; they are embedded in silicon substrates. The orientation and details of the structure of carbon molecules are determined by applying various sample biases to the silicon substrate. As compared with other fullerenes, a bright pentagonal ring with nebulous clusters which represents the cage structure is clearly observed on top of carbon 60 molecules. The bright stripes associated with partitioned curves which depict eight features of asymmetrical C₈₄ molecules are also investigated on Si(1 1 1)-7 × 7 surfaces. The orientations and possible configurations of C₆₀ and C₈₄ are considered in this work. The energy differences for various features of C₆₀ and C₈₄ molecules are estimated and discussed. The corresponding models with respect to each intramolecular feature are proposed and compared with recent theoretical calculation.     

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.     

RHEED intensity oscillation of C60 growth on GaAs substrates

Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C₆₀ layer epitaxial growth on GaAs (1 1 1)B and (1 1 1)A substrates. The frequency of the oscillation coincides well with growth rate of C₆₀ layers, suggesting that C₆₀ layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C₆₀ layer growth on GaAs (1 1 1)B substrates with (2 × 2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C₆₀ molecules. This phenomenon is explained by the model that C₆₀ absorption sites are limited due to As-trimers absorbed on (1 1 1)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (1 1 1)A substrates where no As-trimer is absorbed.     

Insight into STM image contrast of n-tetradecane and n-hexadecane molecules on highly oriented pyrolytic graphite

Two-dimensional ordered patterns of n-tetradecane (n-C₁₄H₃₀) and n-hexadecane (n-C₁₆H₃₄) molecules at liquid/graphite interface have been directly imaged using scanning tunneling microscope (STM) under ambient conditions. STM images reveal that the two different kinds of molecules self-organize into ordered lamellar structures in which alkane chains of the molecules extend along one of three equivalent lattice axes of highly oriented pyrolytic graphite (HOPG) basal plane. For n-C₁₄H₃₀ molecules, the molecular axes are observed to tilt by 60° with respect to inter-lamellar trough lines and the carbon backbones of the alkane chains are perpendicular to the HOPG basal plane in an all-trans conformation. However, for n-C₁₆H₃₄ molecules, the molecular axes are perpendicular to lamellar borders (90°) and the planes of the all-trans carbon skeletons are parallel to the graphite basal plane. The results clearly indicate that outmost hydrogen atoms of the alkane chains dominate atom-scaled features of the STM images. That is, in the case of long-chain alkane molecules, topographic effects dominantly determine STM image contrast of the methylene regions of the alkane chains that are adsorbed on HOPG.     

Adsorption of ethylene on clean and oxygen precovered Cu(110) surfaces

UV photoemission spectroscopy (UPS) with He I and He II radiation is used to study the interaction of C₂H₄ with clean and oxygen precovered Cu(110) surfaces at 90 K. On the clean surface only-bonding of the C₂H₄ molecules is observed whereas preadsorbed oxygen causes a second molecular orbital to be involved in the chemisorption. This result is consistent with the differing behaviour of the work function change during thermal desorption of C₂H₄.     

n-type and p-type double-walled carbon nanotube field-effect transistors based on charge-transfer modulation

Electrical transport properties of double-walled carbon nanotubes (DWNTs) are modulated by encapsulating alkali-metal Cs atom or C₆₀ molecules via a plasma ion-irradiation method. The pristine DWNTs are found to exhibit ambipolar semiconducting behavior due to their small bandgap. In contrast, Cs and C₆₀ encapsulated DWNTs exhibit high performance unipolar n-type and p-type semiconducting behavior since they can operate as an electron donor and acceptor, respectively. Moreover, by controlling the filling level, p–n junction is found to be formed in DWNTs by Cs encapsulation. Our results indicate that DWNTs have great potential as building blocks for various electronic nano devices. PACS 73.63.Fg; 73.63.-b; 61.48.+c     

Thermal desorption states of C<Subscript>60</Subscript> fullerene molecules in polymer matrices

Polymer-C₆₀ fullerene composite coatings are studied using thermal desorption mass spectrometry. It is found that thermal desorption spectra of C₆₀ fullerene molecules can exhibit several resolved peaks (at a specified heating rate) corresponding to thermal desorption states. The relative intensity of the thermal desorption peaks depends on the procedure used for preparing the composite coatings, in particular, on the time of sedimentation of the polymer-fullerene suspension. The occurrence of different stages in thermally stimulated desorption of C₆₀ fullerene molecules is explained by the fact that the fullerene molecules can exist in several phase states characterized by different densities and degrees of ordering in the polymer matrix.     

Conducting properties of planar irradiated and pristine silicon–fullerite–metal structures

Au/C₆₀/p-Si sandwich structures can be easily obtained by evaporation of a thin fullerite (C₆₀) film on a silicon substrate and a thin Au film on top of the C₆₀ film. In this case a C₆₀/p-Si p–n heterojunction appears. Both the dark and photoconductivities of the planar pristine and irradiated Au/C₆₀/p-Si structures were measured as a function of the irradiation fluence. Furthermore, the pressure dependence of these structures was determined. A strong dependence on the irradiation damage was found. PACS 81.05.Tp; 72.80.-r; 72.80.Rj; 72.40.+w; 61.82.Rx     

Imaging C60 fullerite at 4.5 K by scanning tunneling microscopy

Intramolecular contrast of C₆₀ molecules has been observed by scanning tunneling microscopy at 4.5 K on a C₆₀ fullerite thin film sample. This result provides strong evidence for the freezing of the rotational motion of C₆₀ molecules at low temperature as recently proposed by refinement analysis of neutron diffraction data. Different intramolecular patterns are observed. Interpretation of these patterns is suggested by assigning them to carbon ring structures of the C₆₀ molecules in various orientations.     

Measurements of gas desorption from polyethylene-UHMWPE irradiated by 5 MeV electrons

Gas desorption in vacuum from electron irradiated ultra high molecular weight polyethylene (PE) is measured with a high sensible mass quadrupole spectrometer. Measurements are performed in thick PE irradiated with 5 MeV electron beams at doses of the order of tens of kGy. The irradiation modifies the PE molecules producing dehydrogenation, emission of different C _x –H _y groups, C-enrichment and carbon cross-linking processes. Results indicate that the radiation damage depends on the dose and that a significant change of chemical and physical polymer properties is reached for a critical dose of 18 kGy.     

Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Fullerene-C60 and crown ether doped on C60 sensors for high sensitive detection of alkali and alkaline earth cations

Fullerenes are effective acceptor components with high electron affinity for charge transfer. The significant influences of chemical adsorption of the cations on the electrical sensitivity of pristine C₆₀ and 15-(C₂H₄O)₅/C₆₀ nanocages could be the basis of new generation of electronic sensor design. The density functional theory calculation for alkali and alkaline earth cations detection by pristine C₆₀ and 15-(C₂H₄O)₅/C₆₀ nanocages are considered at B3LYP level of theory with 6–31 G(d) basis set. The quantum theory of atoms in molecules analysis have been performed to understand the nature of intermolecular interactions between the cations and nanocages. Also, the natural bond orbital analysis have been performed to assess the intermolecular interactions in detail. Furthermore, the frontier molecular orbital, energy gap, work function, electronegativity, number of transferred electron (∆N), dipole moment as well as the related chemical hardness and softness are investigated and calculated in this study. The results show that the adsorption of cations (M=Na⁺, K⁺, Mg²⁺ and Ca²⁺) are exothermic and the binding energy in pristine C₆₀ nanocage and 15-(C₂H₄O)₅/C₆₀ increases with respect to the cations charge. The results also denote a decrease in the energy gap and an increase in the electrical conductivity upon the adsorption process. In order to validate the obtained results, the density of state calculations are employed and presented in the end as well.