Kurt W. Kolasinski

Spontaneous formation of nanospiked microstructures in germanium by femtosecond laser irradiation

We report a novel phenomenon of spontaneous formation of nearly regular arrays of nanospikes atop conical microstructures by exposing a germanium surface to femtosecond laser pulses in an environment of SF6. Silicon laser texturing has been reported, but no information has been published on laser microtexturing of Ge and in particular the observation of nanospikes atop conical microstructures. The nanospikes are around 2 µm high having a tip radius of 100 nm and are formed atop conical microstructures that are around 5 µm wide and 10 µm tall. The tip radius could be sharpened down to ~10 nm by brief chemical etching. A higher laser fluence and fewer shots favour the formation of nanospikes. By increasing the number of shots at higher fluence the surface morphology changes from conical microstructures to tall straight-walled pillar-like structures and the nanospikes disappear. The surface morphology of germanium has been compared with silicon. The germanium samples turn completely black after laser processing, i.e. they exhibit greatly reduced reflectivity throughout the visible spectrum.

Rotational population and alignment distributions for inelastic scattering and trapping/desorption of NO on Pt(111)

Rotationally resolved experiments on the NO/Pt(111) system explore the mechanisms of inelastic scattering and trapping/desorption. The rotational dynamics associated with these two regimes are markedly different. A neat supersonic NO beam is scattered at normal incidence from a Pt(111) crystal at 375–475 K. The non‐Boltzmann rotational population distribution of the scattered species exhibits considerable rotational excitation beyond the energy available from the incident beam. Thus, a surface vibration to rotational energy transfer mechanism must be operative. The accompanying rotational alignment data reveal that highly excited rotational states exhibit predominantly ‘‘cartwheel’’ motion. In contrast, rotationally excited molecules that desorb from a 553 K Pt(111) surface show a preference for ‘‘helicopter’’ motion. The opposite preferences for rotational alignment in the two dynamical regimes provide insight into the anisotropy of molecule–surface interactions.

Internal-state distribution of recombinative hydrogen desorption from Si(100)

We have measured vibrational‐ and rotational‐state distributions for H2, D2, and HD thermally desorbed from the monohydride phase on Si(100) surfaces using resonance‐enhanced multiphoton ionization detection. The ν=1 to ν=0 population ratio is roughly 20 times higher than that predicted by Boltzmann statistics at the surface temperature, Ts≊780 K. In contrast, the average rotational energies of the desorbed molecules are significantly lower than kTs, exhibit no isotopic dependence within experimental error, and are not significantly different in the ν=0 and ν=1 vibrational states. In the vibrational ground state, we find 〈Erot〉 =345±83 K, 451±77 K, and 332±57 K for H2, HD, and D2, respectively. The degree of vibrational excitation suggests that the H–H interatomic distance in the transition state is elongated compared with the gas‐phase equilibrium bond distance. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high‐impact‐pa...

Recombinative desorption of H2 on Si(100)‐(2×1) and Si(111)‐(7×7): Comparison of internal state distributions

The dynamics of recombinative hydrogen desorption from the Si(100)‐(2×1) and Si(111)‐(7×7) surfaces have been compared using (2+1) resonance‐enhanced multiphoton ionization to probe the desorbed H2. After dosing the surface with disilane (Si2H6), we performed temperature programmed desorption in a quantum‐state‐specific manner. The rovibrational‐state distributions of H2 desorbed from both Si(100)‐(2×1) and Si(111)‐(7×7) are found to be the same within experimental accuracy. The rotational distribution is non‐Boltzmann and has an average energy significantly lower than kTs, where Ts is the surface temperature. In contrast, superthermal energy is observed in the vibrational degree of freedom, and the v=1 to v=0 population ratio is approximately 20 times higher than that predicted by Boltzmann statistics. Our results imply that the details of the recombinative desorption process that affect the product state distribution are remarkably insensitive to the structural differences between the surfaces. We sugge...

The mechanism of Si etching in fluoride solutions

Several mechanisms for (photo)electrochemical etching of Si in aqueous fluoride solutions have been proposed. These models are reviewed and quantitative aspects of the kinetics and energetics of these mechanisms are evaluated. In particular, the roles of HF, F−, HF2−, OH− and H2O in etching both in the dark and under illumination are considered. The presence of a hole in a bulk band (as opposed to a surface state/resonance associated with the Si–H bond or the Si–Si backbond) initiates (photo)electrochemical etching. The sticking coefficient of F−(aq) ions on H-terminated Si increases by 11 orders of magnitude in the presence of this hole. The sticking coefficients of HF(aq) and HF2− (aq) in the course of the rate determining step are also calculated. The possible involvement of abstraction reactions within the overall mechanism is discussed. A modified reaction mechanism, which is consistent with the kinetic and energetic parameters discussed here, is presented.

Rotational alignment of NO desorbing from Pt(111)

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Effects of Stain Etchant Composition on the Photoluminescence and Morphology of Porous Silicon

Stain etching of silicon provides a spontaneous, self-limiting chemical method to produce nanocrystalline silicon films (porous Si). Whereas the existence of etchants capable of producing porous Si has been known for sometime, little has been known concerning how solution composition influences the efficacy of porous Si production and the properties of the resulting films. We demonstrate that the fluoride species may be derived from either HF or acidified NH 4 HF 2 . A range of oxidants may be used as long as their counterions do not lead to precipitation. However, a large positive electrochemical potential is not a sufficient condition for efficient porous Si production. Bubble production, which is deleterious to film homogeneity and long thought to be inherent to the process, can be avoided by the use of transition metal-containing oxidants. Properties of the film, such as morphology, growth rate, porosity, and the wavelength of the photoluminescence maximum, respond to the etchant composition. We observe a blue shift in photoluminescence, which correlates with an increasingly positive electrochemical potential (Eg) of the oxidant. It is argued that Eg plays a role much like wavelength in photoelectrochemical etching and that smaller nanocrystals are produced with more positive values of E 0 .

O2/Pd(111). Clarification of the correspondence between thermal desorption features and chemisorption states

The system O₂/Pd (111) was probed with EELS and thermal desorption spectrometry. The coincidence of 3 peaks in thermal desorption and the O-O region of the EELS spectrum was noted previously and a one-to-one correspondence between the states resolved in EELS and the desorption features was assumed. The authors show that this assumption is incorrect. The lowest binding energy state correlates with the highest frequency vibrational state. The lowest vibrational frequency state acts as a precursor to dissocn. The 2 higher temp. thermal desorption features both arise from the remaining state.

The structure of water on the (0 0 0 1) surface of graphite

The structure of water layers on the close-packed surface of graphite has been studied using electronic structure total energy computations. At all coverages, the water molecules are found to be only physisorbed, lying at least 3.5 A above the graphite and showing no bonding interactions evidenced in electronic orbitals or total electron density. At low coverages, the water shows little preference for orientation or surface site. Increasing coverage by reducing the size of the unit cell leads to a preference for orientation of molecules such that the dipole is parallel to the surface. This is shown to arise from the electrostatic interaction of molecules with their own periodic images. The structures of small water clusters are found to agree with those determined for free, gas-phase clusters. Further increases of coverage lead to the formation of extended ice-like layers.

The Composition of Fluoride Solutions

Aqueous fluoride solutions underpin much of silicon processing because they are used to strip oxide layers and for cleaning. Depending on their composition and reaction conditions they can be used to create either flat, hydrogen-terminated surfaces or nanocrystalline porous silicon. Nonetheless, confusion reigns regarding the identity and quantity of different solution species as a function of the formal concentration of HF and other additives. The literature is critically surveyed, data and results are reanalyzed, and analytical expressions are derived that allow for the calculation of solution species activities. Above 0.1 mol dm - 3 , fluoride solutions are highly nonideal. It is shown that the main constituents of acidic fluoride solutions are H + , F - , HF, HF - 2 , and H 2 F - 3 in the range of 0-2 mol kg - 1 and even as high as 6 mol kg - 1 . In these concentration ranges the derived analytical expressions exhibit excellent and approximate agreement with experimental data, respectively. The nature of the HF species that exists in solution, whether it exists as a contact ion pair or as a solvated molecule, remains in doubt.