Bernd Fruhberger

Controlling surface reactivities of transition metals by carbide formation

Abstract The surface reactivities of transition metals, including the Groups 4–6 early transition metals as well as the late transition metals of the 3d series, can often be modified by the formation of a carbide overlayer. More importantly, the reactivities of carbide-modified surfaces frequently demonstrate strong similarities to those of the Pt-group (Pt, Pd, Ir, Rh, Ru and Os) metals. In this paper, we will summarize our recent surface science investigations of the electronic, structural, and catalytic properties of well-characterized model carbide systems. Using several characteristic surface probing reactions, we will provide experimental evidence for the similar reactivities of the carbide-modified surfaces and the Pt-group metals. We will also discuss the underlying structural and electronic properties that are controlling the reactivities of the carbide-modified surfaces, such as the binding sites and chemical nature of the carbon atoms, the ionicity of the metal–carbon bonds, and the band structure of the transition metal carbides. We will also show examples of experimental attempts to correlate findings from the single crystal model carbide systems to amorphous powder carbide catalysts.

REACTIVITIES OF CARBON- AND NITROGEN-MODIFIED MO(110) : A COMPARISON OF MODIFICATION EFFECTS BY SURFACE AND INTERSTITIAL ADATOMS

The surface properties of carbon‐ and nitrogen‐modified Mo(110) have been investigated using high‐resolution electron energy loss spectroscopy, near‐edge x‐ray absorption fine structure, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and temperature programmed desorption. Exposure of the Mo(110) surface to olefin molecules at 600 K resulted primarily in surface carbon atoms; subsequent annealing to 1200 K produced interstitial/subsurface carbon atoms. Higher concentrations of carbon atoms in the interstitial/subsurface regions could be achieved by repeated dosing/annealing cycles. Exposure to either N2 or NH3 led to the deposition of surface nitrogen atoms. Subsequent annealing of such surfaces resulted in N2 desorption without any detectable production of interstitial/subsurface nitrogen atoms. Reactions with ethylene or cyclopentene as probing molecules demonstrated that the reactivities of carbon‐ and nitrogen‐modified Mo(110) depend strongly on the location of the adatoms. For example,...

Chemical identification using an impedance sensor based on dispersive charge transport

Impedance spectroscopy has been used to identify analytes in semiconducting metallophthalocyanine thin films. Above a critical concentration, the magnitudes of the high frequency conductivity changes are invariant with concentration but distinct for different analytes and can be used for analyte identification. The analyte-induced ac conductivity changes above 5kHz have been converted to frequency shifts in a circuit resonance and used to differentiate methanol, ethanol, and isopropanol vapors in a nitrogen carrier gas. The analyte-induced changes in the conductivity are consistent with analyte-induced changes in the charge relaxation times.

Spectroscopic characterization of thin vanadium carbide films on a vanadium (110) surface: Formation, stability, and reactivities

The surface properties of thin vanadium carbide films, produced on a V(110) surface, have been investigated by using a combination of high‐resolution electron energy‐loss spectroscopy, fluorescence‐yield near‐edge x‐ray absorption spectroscopy (FYNES), and Auger electron spectroscopy (AES). Our results indicate that thin carbide films with a stoichiometry of VC can be produced by exposing V(110) to olefin molecules at 600 K. A comparison of the bulk‐sensitive FYNES data with the relatively surface‐sensitive AES results suggests that the average thickness of the VC film is greater than the penetration depth of Auger electrons for the C(KLL) and V(LMM) transitions. Upon heating to 600–1050 K, the thin VC films most likely undergo a thermally induced clustering process, which is followed by a diffusion of carbon atoms into the bulk V(110). The surface reactivities of the carbide‐modified surfaces are significantly different from those of either clean V(110) or oxygen‐modified V(110), as will be demonstrated ...

Nitride passivation of the interface between high-k dielectrics and SiGe

In-situ direct ammonia (NH3) plasma nitridation has been used to passivate the Al2O3/SiGe interfaces with Si nitride and oxynitride. X-ray photoelectron spectroscopy of the buried Al2O3/SiGe interface shows that NH3 plasma pre-treatment should be performed at high temperatures (300 °C) to fully prevent Ge nitride and oxynitride formation at the interface and Ge out-diffusion into the oxide. C-V and I-V spectroscopy results show a lower density of interface traps and smaller gate leakage for samples with plasma nitridation at 300 °C.

Grazing Incidence Cross-Sectioning of Thin-Film Solar Cells via Cryogenic Focused Ion Beam: A Case Study on CIGSe

Cryogenic focused ion beam (Cryo-FIB) milling at near-grazing angles is employed to fabricate cross-sections on thin Cu(In,Ga)Se2 with >8x expansion in thickness. Kelvin probe force microscopy (KPFM) on sloped cross sections showed reduction in grain boundaries potential deeper into the film. Cryo Fib-KPFM enabled the first determination of the electronic structure of the Mo/CIGSe back contact, where a sub 100 nm thick MoSey assists hole extraction due to 45 meV higher work function. This demonstrates that CryoFIB-KPFM combination can reveal new targets of opportunity for improvement in thin-films photovoltaics such as high-work-function contacts to facilitate hole extraction through the back interface of CIGS.

Quantitative structural characterization of InAs∕GaSb superlattices

Molecular beam epitaxy grown InAs∕GaSb superlattices, containing InSb-like interfacial layers, were analyzed by a combination of x-ray diffraction (XRD) and structural refinement. The superlattice refinement from x rays (SUPREX) method determines with high accuracy the average thicknesses and d spacings of the individual InAs and GaSb layers in addition to standard structural parameters usually obtained by XRD, such as the modulation length (periodicity), average out-of-plane interplanar spacings, and total thickness. The combined SUPREX/XRD experiments show that the absence of certain odd order satellite features in the x-ray data is due to asymmetric and inhomogeneous lattice strain.

A vibrational study of the activation sequence of C–H and C–C bonds of isobutene and 1-butene on Mo(110) and (4 × 4)-C/Mo(110) surfaces

The thermal decomposition pathways of isobutene and 1-butene on both Mo(110) and 4 × 4-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C–H and C–C bonds. On clean Mo(110), the CH2 group of isobutene decomposes upon heating to 150 K, producing either a σ/π-bonded isobutenylidene [(CH3)2CCH] species or a 1,1-di-σ/π-bonded isobutenyl [(CH3)2CC] species. Upon further heating, extensive C–H bond scission occurs to form hydrocarbon fragments which do not contain CH3 or CH2 groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH3)2HCC] by 300 K, which subsequently decomposes via C–C bond scission to generate surface methyl groups. The different activation sequence of the C–H and C–C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.

Implications of Thermal Annealing on the Benzene Vapor Sensing Behavior of PEVA-Graphene Nanocomposite Threads

The effect of thermal treatments, on the benzene vapor sensitivity of polyethylene (co-)vinylacetate (PEVA)/graphene nanocomposite threads, used as chemiresistive sensors, was investigated using DC resistance measurements, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). These flexible threads are being developed as low-cost, easy-to-measure chemical sensors that can be incorporated into smart clothing or disposable sensing patches. Chemiresistive threads were solution-cast or extruded from PEVA and <10% graphene nanoplatelets (by mass) in toluene. Threads were annealed at various temperatures and showed up to 2 orders of magnitude decrease in resistance with successive anneals. Threads heated to ≥80 °C showed improved limits of detection, resulting from improved signal-noise, when exposed to benzene vapor in dry air. In addition, annealing increased the speed of response and recovery upon exposure to and removal of benzene vapor. DSC results showed that the presence of graphene raises the freezing point, and may allow greater crystallinity, in the nanocomposite after annealing. SEM images confirm increased surface roughness/area, which may account for the increase response speed after annealing. Benzene vapor detection at 5 ppm is demonstrated with limits of detection estimated to be as low as 1.5 ppm, reflecting an order of magnitude improvement over unannealed threads.

Ultralow Defect Density at Sub-0.5 nm HfO2/SiGe Interfaces via Selective Oxygen Scavenging

The superior carrier mobility of SiGe alloys make them a highly desirable channel material in complementary metal-oxide-semiconductor (CMOS) transistors. Passivation of the SiGe surface and the associated minimization of interface defects between SiGe channels and high- k dielectrics continues to be a challenge for fabrication of high-performance SiGe CMOS. A primary source of interface defects is interfacial GeO x. This interfacial oxide can be decomposed using an oxygen-scavenging reactive gate metal, which nearly eliminates the interfacial oxides, thereby decreasing the amount of GeO x at the interface; the remaining ultrathin interlayer is consistent with a SiO x-rich interface. Density functional theory simulations demonstrate that a sub-0.5 nm thick SiO x-rich surface layer can produce an electrically passivated HfO2/SiGe interface. To form this SiO x-rich interlayer, metal gate stack designs including Al/HfO2/SiGe and Pd/Ti/TiN/nanolaminate (NL)/SiGe (NL: HfO2-Al2O3) were investigated. As compared to the control Ni-gated devices, those with Al/HfO2/SiGe gate stacks demonstrated more than an order of magnitude reduction in interface defect density with a sub-0.5 nm SiO x-rich interfacial layer. To further increase the oxide capacitance, the devices were fabricated with a Ti oxygen scavenging layer separated from the HfO2 by a conductive TiN diffusion barrier (remote scavenging). The Pd/Ti/TiN/NL/SiGe structures exhibited significant capacitance enhancement along with a reduction in interface defect density.