Hydrogenated Si Surface Research Articles

Effect of the surface coverage of an alkyl carboxylic acid monolayer on waterborne and cellular uptake behaviors for silicon quantum dots

This article reports the development of highly waterborne silicon quantum dots (Si QDs) terminated with a reactive group for grafting of biomolecules. Hydrogen-terminated QDs were prepared by thermal disproportionation of amorphous hydrogen silsesquioxane derived from triethoxysilane followed by hydrofluoric etching. Next, the hydrogenated Si surfaces were exposed to 10-undecenoic acid at different temperatures in Ar atmosphere, yielding the termination of the QDs with a carboxyl group. The thermal hydrosilylation of 10-undecenoic acid yielded the termination of the QDs with a carboxyl group. An increase in molecular coverage of an undecanoic acid (UA) monolayer resulted in both the enhanced increase of zeta-potential in a negative direction for a greater water-dispersity and the increase of absolute quantum yield (QY) of photoluminescence (PL). PLQY improved for ~ 1% to 26% with increasing UA coverage. We assessed the molecular interaction between the UA-SiQDs and HeLa cells by means of cellular uptake experiments using the QDs with different UA coverages. Results showed that the QDs with the highest dispersity in water were not internalized in the cells under confocal fluorescence microscopic observation. In contrast, the QDs with lower coverage of UA monolayer were internalized by endocytosis when incubated with HeLa cells. This contrasting observation opens the possibility of successfully preparing carboxy-capped SiQDs that do not allow cellular uptake but are targeted to specific cells by appropriate conjugation with biomolecules.

Quantitative Analysis of Nanorough Hydrogenated Si(111) Surfaces through Vibrational Spectral Assignment by Periodic DFT Calculations

In this work, we use periodic density functional theory (periodic DFT) to rigorously assign vibrational spectra measured on nanorough wet-processed hydrogenated Si(111) surfaces. We compare Si(111)-(1 × 1) surfaces etched by dilute HF and NH4F, featuring two vibrational patterns that systematically appear together. They are attributed to vibrations observed on vicinal surfaces featuring 112̅ and 1̅1̅2 steps terminated with monohydrides and dihydrides, respectively. For the first time, we fully assign vibration patterns of realistic silicon surfaces with variable nanoroughness directly by periodic DFT simulations involving contributions from isolated species but also contributions from highly coupled species forming standing waves. This work opens the path to a better quantitative characterization of imperfect and nanorough Si(111) surfaces from vibrational spectra.

Visualizing the effects of plasma-generated H atoms in situ in a transmission electron microscope

The radicals and atoms generated by a plasma have the effect, among others, of changing the surface energies of materials, which allows one to prepare nano-objects that would not stabilise in other conditions. This is the case of the Sn catalysed silicon nanowires (NWs) we present in this paper: without plasma, the liquid Sn at the top of NWs is unstable (because Sn naturally wets the Si) so that no growth is allowed, while in presence of the H atoms generated by the plasma, the balance of surface energies is drastically changed; the Sn droplet stabilises and can be used efficiently by the vapour-liquid-solid (VLS) mechanism of growth. Thus, if one wants to study the growth mechanisms of such NWs in situ in the transmission electron microscope (TEM), one has to adapt a plasma system on the TEM. This is precisely what was done at École polytechnique on the NanoMAX environmental TEM. The paper reports on the plasma effects, on the catalyst and on NW growth, recorded in situ in real time, at atomic resolution. The results are discussed in the light of density functional calculations of bare and hydrogenated Si surface energies.

Probing the electronic properties of the electrified silicon/water interface by combining simulations and experiments

Silicon (Si) is broadly used in electrochemical and photoelectrochemical devices, where the capacitive and Faradaic reactions at the Si/water interfaces are critical for signal transduction or noise generation. However, probing the electrified Si/water interface at the microscopic level remains a challenging task. Here we focus on hydrogenated Si surfaces in contact with water, relevant to transient electronics and photoelectrochemical modulation of biological cells and tissues. We show that by carrying out first-principles molecular dynamics simulations of the Si(100)/water interface in the presence of an electric field we can realistically correlate the computed flat-band potential and tunneling current images at the interface with experimentally measured capacitive and Faradaic currents. Specifically, we validate our simulations in the presence of bias by performing pulsed chronoamperometry measurements on Si wafers in solution. Consistent with prior experiments, our measurements and simulations indicate the presence of voltage-dependent capacitive currents at the interface. We also find that Faradaic currents are weakly dependent on the applied bias, which we relate to surface defects present in newly prepared samples.

Theoretical Analysis of Si2H6 Adsorption on Hydrogenated Silicon Surfaces for Fast Deposition Using Intermediate Pressure SiH4 Capacitively Coupled Plasma

The rapid and uniform growth of hydrogenated silicon (Si:H) films is essential for the manufacturing of future semiconductor devices; therefore, Si:H films are mainly deposited using SiH4-based plasmas. An increase in the pressure of the mixture gas has been demonstrated to increase the deposition rate in the SiH4-based plasmas. The fact that SiH4 more efficiently generates Si2H6 at higher gas pressures requires a theoretical investigation of the reactivity of Si2H6 on various surfaces. Therefore, we conducted first-principles density functional theory (DFT) calculations to understand the surface reactivity of Si2H6 on both hydrogenated (H-covered) Si(001) and Si(111) surfaces. The reactivity of Si2H6 molecules on hydrogenated Si surfaces was more energetically favorable than on clean Si surfaces. We also found that the hydrogenated Si(111) surface is the most efficient surface because the dissociation of Si2H6 on the hydrogenated Si(111) surface are thermodynamically and kinetically more favorable than those on the hydrogenated Si(001) surface. Finally, we simulated the SiH4/He capacitively coupled plasma (CCP) discharges for Si:H films deposition.

Electron paramagnetic resonance calculations for hydrogenated Si surfaces

Electron paramagnetic resonance (EPR) signatures, more specifically the elements of the electronic $g$ tensor, are calculated within density functional theory for hydrogenated Si(111), Si(001), Si(113), Si(114), $\mathrm{Si}(11\overline{2})$, and Si(110) surfaces. Thereby both perturbation theory and a more sophisticated Berry phase technique are applied. Specific defects on different surface orientations are shown to reproduce the resonances at $\overline{g}=2.0043$ and $\overline{g}=2.0052$ measured for hydrogenated microcrystalline silicon: The latter value is argued here to originate from regions with low hydrogen coverage; the resonance at $\overline{g}=2.0043$ is shown to appear in positions with dihydride environment, where an H atom is directly bound to the silicon dangling-bond atoms. A third group of EPR signals with considerably larger $\overline{g}$ values between 2.006 and 2.009 is predicted for highly symmetric dangling bonds resembling single dangling-bond defects in silicon bulk material. As the exact value depends strongly on local strain, this type of defect can explain a less intense signal with large $g$ strain observed in microcrystalline as well as in amorphous material.

Hydrogenation of silicene films grown on Ag(111)

We have investigated the effect of hydrogen interaction with silicene monolayer and multilayer films grown on Ag(111) by means of scanning tunneling microscopy, low energy diffraction and high-resolution electron energy loss spectroscopy. On monolayer silicene, we demonstrate that the recently reported hydrogenated γ-(3×3) is indeed observed, but is found often in coexistence with the usual “flower” pattern of the clean silicene-(3×3), also labeled α-(3×3). In the vibrational spectra, features related to SiH vibrational modes show shifts towards lower energies as compared to hydrogenated Si surfaces. This substantial bond softening is indicative of a partial charge filling of the bonding or anti-bonding orbitals of the SiH bonds as compared to Si(111)H. Indirectly, this is an evidence of the hybridization of monolayer silicene with the Ag(111) substrate. At multilayer coverage, the situation is clearly different. Indeed, hydrogen strongly perturbs the √3×√3 reconstruction of the multilayer film, leading to its local dissolution and to the formation of protrusions due to clustering of atoms. The vibrational spectrum of the √3×√3 silicene closely resembles that of a highly hydrogenated Si surfaces, with the simultaneous presence of SiH, SiH2 and SiH3 bonds.

Visible to Near-Infrared Sensitization of Silicon SubstratesviaEnergy Transfer from Proximal Nanocrystals: Further Insights for Hybrid Photovoltaics

We provide a unified spectroscopic evidence of efficient energy transfer (ET) from optically excited colloidal nanocrystal quantum dots (NQDs) into Si substrates in a broad range of wavelengths: from visible (545 nm) to near-infrared (800 nm). Chemical grafting of nanocrystals on hydrogenated Si surfaces is achieved via amine-modified carboxy-alkyl chain linkers, thus ensuring complete surface passivation and accurate NQD positioning. Time-resolved photoluminescence (PL) has been measured for a set of CdSe/ZnS and CdSeTe/ZnS NQDs of various sizes and compositions grafted on Si and SiO2 substrates. The measured acceleration of the PL decays on Si substrates is in good agreement with theoretical expectations based on the frequency-dependent dielectric properties of Si and NQD-Si separation distances. A comparative analysis reveals separate contributions to ET coming from the nonradiative (NRET) and radiative (RET) channels: NRET is a dominant mechanism for proximal NQDs in the middle of the visible range and becomes comparable with RET toward near-infrared wavelengths. The broad range over which the ET efficiency is estimated to be at the level of ∼90% further supports the concept that hybrid nanocrystal/silicon thin-film photovoltaic devices could efficiently harvest solar energy across the entire spectrum of wavelengths.

Simple fabrication of nanostructured silicon and photoluminescence

Porous Si (PS) was fabricated simply by electrochemical anodic etching of a Si wafer. By a combination of SEM, EDX and infrared spectrum measurements hydrogen-terminated crystalline Si surfaces were identified. Transparent Si nanocolloids were obtained via thermally-initiated hydrosilation of the hydrogenated Si surfaces reaction with unsaturated 1-undecene. In contrast to weak luminescence in the hydrogenated PS dispersed in toluene, intense luminescence was observed in the Si nanocolloid. In the Si nanocolloid, both strong luminescence and long-term stability can be correlated with alkylated passivation, as formation of Si–C bonds identified in infrared spectrum. Additionally, the oxidation effect was noticeable in affecting the luminescence of nano-Si during the process.

Spectroscopic evidence for nonradiative energy transfer between colloidal CdSe/ZnS nanocrystals and functionalized silicon substrates

We present the fabrication and properties of hybrid structures consisting of a monolayer of colloidal CdSe nanocrystals grafted on hydrogenated Si surfaces via amine modified carboxy-alkyl chain linkers. The wet chemical preparation ensures that Si surfaces are fully passivated with a negligible number of nonradiative surface state defects and that the separation between nanocrystals and Si is tightly controlled. An eightfold decrease in photoluminescence lifetime of nanocrystals on Si is observed as compared to glass. A quantitative analysis reveals that the nonradiative transfer from nanocrystals to Si is 65% efficient, demonstrating the potential of such hybrids for practical photovoltaic devices.

Electronic Properties and pH Stability of Si(111)/Alkyl Monolayers

The electronic properties of hydrogenated and alkyl-grafted silicon surfaces have been investigated by photoluminescence, surface photovoltage and electrochemical capacitance measurements, which are sensitive methods for monitoring the presence of electronic defects. On p-type silicon, the electronic quality of the interface was found to depend critically on the surface preparation and was studied with special care. The thermal grafting of one monolayer of linear organic chains on atomically flat silicon allows the formation of stable alkyl-grafted p-Si surfaces, exhibiting a very low density of recombination centers comparable to those measured on hydrogenated silicon surfaces. Photoluminescence measurements show that the high electronic quality of the grafted surfaces is preserved on a month scale in air, indicating a much more efficient long term passivation by comparison with hydrogenated Si surfaces. The stability of the silicon/organic layer interface in aqueous buffer solution has been assessed by in situ photoluminescence and surface photovoltage measurements. Some degradation of the electronic properties was evidenced during exposure in basic solution of pH > 9.

Electronic Characterization of Si(100)-Bound Alkyl Monolayers Using Kelvin Probe Force Microscopy

Hydrogen-terminated and alkyl-chain (C18H37)-terminated Si(100) surfaces with different doping levels have been characterized using Kelvin probe force microscopy. n- and p-doped Si(100) and lateral p++n and n++p silicon junctions were hydrogenated in dilute HF solution, followed with a self-assembly deposition of organic molecules by thermally activated free-radical reaction between CC and Si−H. The surface band bending following the two different chemical treatments was almost identical for both p-type silicon (∼0.7 eV, with a surface charge of 9.4 ± 0.5 × 1011/cm2) and n-type silicon (0.6 eV, with a surface charge of 8.7 ± 0.5 × 1011/cm2). These results indicate that the self-assembly of the C18H37 monolayer on a Si (100) surface results in electrical properties similar to those of a hydrogenated Si surface, with the advantage of longer stability in an ambient environment. The hydrogen-terminated and alkyl-chain-terminated surface do differ, however, in the surface dipole, which is lower by ∼0.6 eV for the latter, a value deduced from both the measurements and independent first principles electronic structure calculations. This dipole change is essentially due to the change in bond dipole associated with the replacement of Si−H bonds by Si−C bonds and the dipole associated with the methyl group.

Tuning the redox potential in molecular monolayers covalently bound to H–Si(100) electrodes via distinct C–C tethering arms

The spatial self-organization of molecular species on an Si oriented surface can be less ideal than that of an SAM on a metal, likely affecting the electronic structure of the resulting hybrids and their electrochemical response as electrodes in solution. In order to investigate such effects, a series of molecular precursors was investigated, consisting of three substituted ferrocenes with a lateral C–C group fully saturated (ethylferrocene) or with a single (vinylferrocene) or double unsaturation (ethynylferrocene). The corresponding functionalized Si(100) wafers were produced following new or literature recipes, starting from hydrogenated Si surfaces. The relationship between the degree of unsaturation in the anchored arm of each adduct and its electronic structure and electrochemical behaviour was investigated by comparing experimental (XPS, electrochemical) and ab initio results of the redox potentials in the series. Density functional theory (DFT) was applied, with inclusion of solute–solvent interactions. Different bond arrangements of the C–C arm with Si surface dimer atoms have been produced theoretically within the series of ferrocenes. Distinct values of redox potentials were displayed by the hybrids, which can be consistently related to the structural differences presented. In fact, measured and computed potentials showed a very satisfactory match only for specific adduct isomers, providing strong indications that the carbon–carbon unsaturation initially present in the anchoring arm is preserved upon addition reaction, an unprecedented result. The demonstrated tunability of a well-defined switching potential in these molecules on silicon can be beneficial to the development of Si-based memory devices.

Modelling of Plasma Surface Interaction

Molecular dynamics (MD) simulations are used to investigate chemical reactions that take place on surfaces exposed to ion bombardment and neutral radicals extracted from plasmas. Such chemical reactions occur in a nano-meter-scale thin top layer of the material and their characteristics significantly differ from those of ordinary chemical reactions in thermodynamical equilibrium. As applications of MD simulations to microscale fabrication and thin film formation processes, recent MD simulation results on organic polymer etching by hydrogen/nitrogen plasmas and surface diffusion of Si on highly hydrogenated Si surfaces are reviewed.

Molecular dynamics simulation of microcrystalline Si deposition processes by silane plasmas

Molecular dynamics (MD) simulations have been performed to examine enhanced surface diffusion of Si adatoms during silane(SiH 4)-based plasma enhanced chemical vapor deposition (PECVD) processes. Such high surface diffusion, if it actually takes place, has been known to account for the growth of microcrystalline silicon( μc-Si) in the PECVD process. Focused in the present study is the motion of a silicon (Si) adatom on a hydrogenated Si surface and the surface diffusion coefficient of Si adatoms on the fully hydrogenated (111) Si surface at 600 K was evaluated from MD simulations. The obtained diffusion coefficient is much larger than those of typical clean Si surfaces known in the literature. The interatomic potential functions for Si–H systems used for the simulations, which we have developed for this study based on ab initio calculations of the interatomic energies, are also presented.

Engineering of Si surfaces by electrochemical grafting of p-nitrobenzene molecules

The changes of the band bending and of the nonradiative (nr) surface recombination are investigated by use of photovoltage and photoluminescence techniques during the electrochemical deposition of p-nitrobenzene molecules on atomically flat and rough hydrogenated as well as on chemically oxidized Si(111) surfaces. A simple and well-reproducible procedure has been developed for electrochemical grafting of organic molecules on hydrogenated Si surfaces in aqueous electrolytes. The grafting of a monolayer of p-nitrobenzene molecules on atomically flat p-Si(111):H surfaces induces a change of the band bending of about 0.1 eV and the amount of nr surface defects, Ns, is only slightly increased by a factor of about 3 (Ns<1011 cm−2) with respect to the hydrogenated Si surface. The role of the formation of radicals for the engineering of Si surfaces is discussed.

Defect transformation under growth of submonolayer oxides on silicon surfaces at low temperatures

The defect transformation under growth of submonolayer oxides on silicon surfaces is studied in situ on reconstructed Si(100)-2×1 and Si(111)-7×7 surfaces and on hydrogenated Si surfaces of Si nanocrystallites. The methods of the quenching of the photoluminescence of bulk c-Si due to surface recombination and of the infrared free carriers absorption in mesoporous Si are used. It is shown that dangling bonds at Si–Si dimers are not recombination active. Non-radiative surface defects are generated during the earliest stages of oxidation. The generation of hole traps is tightly related to an increase of physisorbed H 2O at hydrogenated Si surfaces during the native oxidation.

Annihilation of nonradiative defects on hydrogenated silicon surfaces under pulsed-laser irradiation

Annihilation of nonradiative recombination defects on hydrogenated Si surfaces has been found after irradiation with XeCl laser pulses of energies below the melting threshold. A decrease of the total numbers of the stretched Si–Hx (x=1,2,3) bonds and an increase of the relative part of the Si–H bonds of the surface coverage have been established simultaneously by Fourier transform infrared spectroscopy. The drop of the number of surface nonradiative defects despite the efficient hydrogen desorption is explained by the mechanism of bonding of Si dangling bonds under the high electronic excitation induced by the XeCl laser pulse.

Effect of local surface structure on electronic properties of hydrogenated silicon surfaces

Morphology and electronic properties of hydrogenated Si surfaces are studied by scanning tunneling microscopy and by methods of surface photovoltage and photoluminescence (PL), respectively. The microscopic roughness on the nm scale is more important than the macroscopic roughness of a hydrogenated Si surface for the densities of nonradiative surface defects and hole traps. The adsorption of water or oxygen molecules on a hydrogenated Si surface leads to an increase of the intensity of interband PL of c-Si which is controlled by surface nonradiative recombination. This effect is interpreted as a decrease of the recombination cross section of a nonradiative surface defect due to charge trapped at the adsorbed molecules.

On the origin of the positive charge on hydrogenated Si surfaces and its dependence on the surface morphology

Chemically and electrochemically hydrogenated Si surfaces are investigated by surface photovoltage technique, scanning tunneling microscopy, and Fourier transform infrared spectroscopy. The surface states are predominantly of the donor type which can act as fixed positive surface charge ( Q f) as well as fast surface states depending on the surface morphology and the applied field voltage. The donor type surface states on hydrogenated Si surfaces can be related to water molecules adsorbed at surface imperfections. The lowest values of Q f and of the density of surface states in the minimum D min it (10 10 cm −2 and 10 10 eV −1 cm −2, respectively) are reached on macroscopically rough hydrogenated surfaces which are locally very smooth.