Hydrogen-terminated Silicon Research Articles

Dynamic Chemically Driven Dewetting, Spreading, and Self-Running of Sessile Droplets on Crystalline Silicon.

A chemically driven dewetting effect is demonstrated using sessile droplets of dilute hydrofluoric acid on chemically oxidized silicon wafers. The dewetting occurs as the thin oxide is slowly etched by the droplet and replaced by a hydrogen-terminated surface; the result of this is a gradual increase in the contact angle of the droplet with time. The time-varying work of adhesion is calculated from the time-varying contact angle; this corresponds to the changing chemical nature of the surface during dewetting and can be modeled by the well-known logistic (sigmoid) function often used for the modeling of restricted growth, in this case, the transition from an oxidized surface to a hydrogen-terminated silicon surface. The observation of the time-varying contact angle allows one to both measure the etch rate of the silicon oxide and estimate the hydrogenation rate as a function of HF concentration and wafer type. In addition to this, at a certain HF concentration, a self-running droplet effect is observed. In contrast, on hydrogen-terminated silicon wafers, a chemically induced spreading effect is observed using sessile droplets of nitric acid. The droplet spreading can also be modeled using a logistical function, where the restricted growth is the transition from hydrogen-terminated to a chemically induced oxidized silicon surface. The chemically driven dewetting and spreading observed here add to the methods available to study dynamic wetting (e.g., the moving three-phase contact line) of sessile droplets on surfaces. By slowing down chemical kinetics of the wetting, one is able to record the changing profile of the sessile droplet with time and gather information concerning the time-varying surface chemistry. The data also indicates a chemical interface hysteresis (CIH) that is compared to contact angle hysteresis (CAH). The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena.

Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-Decorated Silicon Photocathodes by the Temperature and Time of Electroless Pt Deposition.

The electroless deposition of Pt nanoparticles (NPs) on hydrogen-terminated silicon (H-Si) surfaces is studied as a function of the temperature and the immersion time. It is demonstrated that isolated Pt structures can be produced at all investigated temperatures (between 22 and 75 °C) for short deposition times, typically within 1-10 min if the temperature is 45 °C or less than 5 min at 75 °C. For longer times, dendritic metal structures start to grow, ultimately leading to highly rough interconnected Pt networks. Upon increasing the temperature from 22 to 75 °C and for an immersion time of 5 min, the average size of the observed Pt NPs monotonously increases from 120 to 250 nm, and their number density calculated using scanning electron microscopy decreases from (4.5 ± 1.0) × 108 to (2.0 ± 0.5) × 108 Pt NPs cm-2. The impact of both the morphology and the distribution of the Pt NPs on the photoelectrocatalytic activity of the resulting metallized photocathodes is then analyzed. Pt deposited at 45 °C for 5 min yields photocathodes with the best electrocatalytic activity for the hydrogen evolution reaction. Under illumination at 33 mW cm-2, this optimized photoelectrode shows a fill factor of 45%, an efficiency (η) of 9.7%, and a short-circuit current density (|Jsc|) at 0 V versus a reversible hydrogen electrode of 15.5 mA cm-2.

Electron Transport Behavior Analysis of Bis(terpyridine) Metal Complex Wires on Electrodes

Molecular wires with rapid and smooth charge transport ability are desired to achieve high performance and low energy consumption of molecular electronic devices. Various kinds of molecular wires with superior electron transport abilities have been reported. They exhibit the tiny dependencies of electron transfer rate constants on the wire lengths, in other words the small attenuation factors, β. We have reported that bis(terpyridine)metal complex [M(tpy)2] (M=Fe, Co, tpy=2,2’:6’,2’’-terpyridine) oligomer wires show the small β values because M[(tpy)2] unit works as an electron hopping site and promotes intrawire electron transport. The quantitative evaluation of charge transport is important to develop superior electron transport system. In this research, we prepared two types of gold electrode-[Fe(tpy)2]-ferrocene molecular arrays as the simplest electron transport model of [M(tpy)2] wire system. The prepared arrays were characterized using cyclic voltammetry and scanning tunneling microscope. The electron transfer between the gold electrode and the terminal ferrocene moiety via [Fe(tpy)2] was evaluated by the potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS). The analysis of Tafel plots derived from PSCA based on Marcus theory revealed the greater electron transfer rate constant and smaller reorganization energy than a ferrocene-terminated alkanethiol self-assembled monolayer system. The parameters extracted by the fitting of the observed impedance spectra using an electric circuit model suggested the additional factor for the rapid electron transport of [Fe(tpy)2] system. On the other hand, in the present electronics field, semiconductors are used as major materials for electronic devices. Therefore, the investigation of the electron transport behaviors of Fe(tpy)2 wires on a semiconductor electrode will contribute to develop molecular devices based on the present electronics. Ferrocene- and triarylamine-terminated [Fe(tpy)2] wires were prepared on hydrogen-terminated silicon electrodes using the stepwise coordination method. Their electron transport behaviors between the terminal redox species and silicon electrodes via [Fe(tpy2)] wires were analyzed based on the cyclic voltammograms recorded at various scan rates. The ferrocene-terminated wire gave the redox wave derived from Fc+/Fc with largely-separated oxidation and reduction peaks indicating slow electron transfer while the triarylamine-terminated wire exhibited the reversible redox couple of triarylamine cation radical/triarylamine derived from the rapid electron transport. In addition, the separation of oxidation and reduction peak potentials of Fc+/Fc redox wave showed the linear dependency on the natural logarithm of scan rates at the high scan rate region whereas the that of triarylamine redox couple did not. These different electron transport phenomena were discussed based on the band structure of silicon electrode and the energy levels of wire components. At the redox potential of ferrocene moiety, the band structure of silicon is largely bended, and the electron injection is prevented. In the case of triarylamine moiety, the band deformation is insignificant. In addition, the applied potential is close to the HOMO level of Fe(tpy)2, and might allow the smooth electron injection from the wire to the electrode.

Electron Transport Behavior Analysis of Bis(terpyridine) Metal Complex Wires on Electrodes

Bis(terpyridine)iron(II) complex (Fe(tpy)2) wires terminated with redox active molecules were sequentially constructed on gold and hydrogen-terminated silicon electrodes. Potential-step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS) were performed to study the electron transfer of ferrocene-Fe(tpy)2 arrays on gold surfaces. The obtained Tafel plots and impedance spectra revealed greater first-order electron transfer rate constant than a ferrocene-terminated alkanethiol self-assembled monolayer and parameters contributing the smooth electron transport. On silicon electrodes, ferrocene-terminated Fe(tpy)2 wires showed slow electron transport between the ferrocene and the electrode while triarylamine-terminated Fe(tpy)2 wires displayed rapid electron transport. The difference of electron transport behaviors was discussed based on the band structure of silicon electrode and the HOMO level of terminal redox units.

Effective Method for Micro-Patterning Arene-Terminated Monolayers on a Si(111) Electrode.

Microstructured electrodes are significant to modern electrochemistry. A representative aromatic group, 4-ferrocenylphenyl one, was covalently bound to a micropatterned silicon electrode via the arylation of a hydrogen-terminated silicon(111) surface formed selectively on a Si wafer. Starting from a silicon(100)-on-insulator (SOI) wafer, the aromatic monolayer was attached sequentially by spin-coating a resist, electron beam lithography, Cr/Au deposition, lift-off, anisotropic etching with aqueous KOH solution, and Pd-catalyzed arylation. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) are used to characterize the coupling reaction between 4-ferrocenyl group and silicon substrate, and to confirm performance of the final modified microsized electrode. These data show that this synthetic protocol gives chemically well-defined and robust functionalized monolayers on a silicon semiconducting surface with a small electrode.

Reactions of Persistent Carbenes with Hydrogen-Terminated Silicon Surfaces.

Surface passivation has enabled the development of silicon-based solar cells and microelectronics. However, a number of emerging applications require a paradigm shift from passivation to functionalization, wherein surface functionality is installed proximal to the silicon surface. To address this need, we report here the use of persistent aminocarbenes to functionalize hydrogen-terminated silicon surfaces via Si-H insertion reactions. Through the use of model compounds (H-Si(TMS)3 and H-Si(OTMS)3), nanoparticles (H-SiNPs), and planar Si(111) wafers (H-Si(111)), we demonstrate that among different classes of persistent carbenes, the more electrophilic and nucleophilic ones, in particular, a cyclic (alkyl)(amino)carbene (CAAC) and an acyclic diaminocarbene (ADAC), are able to undergo insertion into Si-H bonds at the silicon surface, forming persistent C-Si linkages and simultaneously installing amine or aminal functionality in proximity to the surface. The CAAC (6) is particularly notable for its clean insertion reactivity under mild conditions that produces monolayers with 21 ± 3% coverage of Si(111) atop sites, commensurate with the expected maximum of ∼20%. Atomic force and transmission electron microscopy, nuclear magnetic resonance, X-ray photoelectron, and infrared spectroscopy, and time-of-flight secondary ion mass spectrometry provided evidence for the surface Si-H insertion process. Furthermore, computational studies shed light on the reaction energetics and indicated that CAAC 6 should be particularly effective at binding to silicon dihydride, trihydride, and coupled monohyride motifs, as well as oxidized surface sites. Our results pave the way for the further development of persistent carbenes as universal ligands for silicon and potentially other nonmetallic substrates.

Fluorinated alkyne-derived monolayers on oxide-free silicon nanowires via one-step hydrosilylation

Passivation of oxide-free silicon nanowires (Si NWs) by the formation of high-quality fluorinated 1-hexadecyne-derived monolayers with varying fluorine content has been investigated. Alkyl chain monolayers (C16H30−xFx) with a varying number of fluorine substituents (x=0, 1, 3, 9, 17) were attached onto hydrogen-terminated silicon (SiH) surfaces with an effective one-step hydrosilylation. This surface chemistry gives well-defined monolayers on nanowires that have a cylindrical core–shell structure, as characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and static contact angle (SCA) analysis. The monolayers were stable under acidic and basic conditions, as well as under extreme conditions (such as UV exposure), and provide excellent surface passivation, which opens up applications in the fields of field effect transistors, optoelectronics and especially for disease diagnosis.

Photochemical Preparation of Alkoxy Self-assembled Monolayers on Si from 1,2-Epoxyalkane Molecules

We have immobilized 1,2-epoxydodecane (EDD) on hydrogen-terminated silicon (H-Si) substrates by UV photochemical grafting. The surface properties of the fabricated samples showed the formation of h...

Hydrogen-terminated mesoporous silicon monoliths with huge surface area as alternative Si-based visible light-active photocatalysts

Mesoporous silicon structures classified into two structural models show photocatalytic potentials with hydrides as one of the two pathways.

Inherent substrate-dependent growth initiation and selective-area atomic layer deposition of TiO2 using “water-free” metal-halide/metal alkoxide reactants

Titanium dioxide atomic layer deposition (ALD) is shown to proceed selectively on oxidized surfaces with minimal deposition on hydrogen-terminated silicon using titanium tetrachloride (TiCl4) and titanium tetra-isopropoxide [Ti(OCH(CH3)2)4, TTIP] precursors. Ex situ x-ray photoelectron spectroscopy shows a more rapid ALD nucleation rate on both Si–OH and Si–H surfaces when water is the oxygen source. Eliminating water delays the oxidation of the hydrogen-terminated silicon, thereby impeding TiO2 film growth. For deposition at 170 °C, the authors achieve ∼2 nm of TiO2 on SiO2 before substantial growth takes place on Si–H. On both Si–H and Si–OH, the surface reactions proceed during the first few TiCl4/TTIP ALD exposure steps where the resulting products act to impede subsequent growth, especially on Si–H surfaces. Insight from this work helps expand understanding of “inherent” substrate selective ALD, where native differences in substrate surface reaction chemistry are used to promote desired selective-area growth.

Low-Temperature Synthesis of a TiO2/Si Heterojunction.

The classical SiO2/Si interface, which is the basis of integrated circuit technology, is prepared by thermal oxidation followed by high temperature (>800 °C) annealing. Here we show that an interface synthesized between titanium dioxide (TiO2) and hydrogen-terminated silicon (H:Si) is a highly efficient solar cell heterojunction that can be prepared under typical laboratory conditions from a simple organometallic precursor. A thin film of TiO2 is grown on the surface of H:Si through a sequence of vapor deposition of titanium tetra(tert-butoxide) (1) and heating to 100 °C. The TiO2 film serves as a hole-blocking layer in a TiO2/Si heterojunction solar cell. Further heating to 250 °C and then treating with a dilute solution of 1 yields a hole surface recombination velocity of 16 cm/s, which is comparable to the best values reported for the classical SiO2/Si interface. The outstanding performance of this heterojunction is attributed to Si-O-Ti bonding at the TiO2/Si interface, which was probed by angle-resolved X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that Si-H bonds remain even after annealing at 250 °C. The ease and scalability of the synthetic route employed and the quality of the interface it provides suggest that this surface chemistry has the potential to enable fundamentally new, efficient silicon solar cell devices.

Selective Catalytic Electroreduction of CO2 at Silicon Nanowires (SiNWs) Photocathodes Using Non-Noble Metal-Based Manganese Carbonyl Bipyridyl Molecular Catalysts in Solution and Grafted onto SiNWs

The electrocatalytic reduction of CO2 to CO in hydroorganic medium has been investigated at illuminated (λ > 600 nm; 20 mW cm–2) hydrogen-terminated silicon nanowires (SiNWs–H) photocathodes using three Mn-based carbonyl bipyridyl complexes as homogeneous molecular catalysts ([Mn(L) (CO)3(CH3CN)](PF6) and [Mn(bpy) (CO)3Br] with L = bpy = 2,2′-bipyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine). Systematic comparison of their cyclic voltammetry characteristics with those obtained at flat hydrogen-terminated silicon and traditional glassy carbon electrodes (GCE) enabled us to demonstrate the superior catalytic efficiency of SiNWs–H in terms of cathodic photocurrent densities and overpotentials. For example, the photocurrent densities measured at −1.0 V vs SCE for [Mn(bpy) (CO)3(CH3CN)](PF6) at SiNWs–H exceeded 1.0 mA cm–2 in CO2-saturated CH3CN + 5% v/v H2O, whereas almost zero current was measured at this potential at GCE. Such characteristics have been supported by the energetic diagrams built for the d...

"Click" Patterning of Self-Assembled Monolayers on Hydrogen-Terminated Silicon Surfaces and Their Characterization Using Light-Addressable Potentiometric Sensors.

Two potential strategies for chemically patterning alkyne-terminated self-assembled monolayers (SAMs) on oxide-free silicon or silicon-on-sapphire (SOS) substrates were investigated and compared. The patterned surfaces were validated using a light-addressable potentiometric sensor (LAPS) for the first time. The first strategy involved an integration of photolithography with "click" chemistry. Detailed surface characterization (i.e. water contact angle, ellipsometry, AFM, and XPS) and LAPS measurements showed that photoresist processing not only decreases the coverage of organic monolayers but also introduces chemically bonded contaminants on the surfaces, thus significantly reducing the quality of the SAMs and the utility of "click" surface modification. The formation of chemical contaminants in photolithography was also observed on carboxylic acid- and alkyl-terminated monolayers using LAPS. In contrast, a second approach combined microcontact printing (μCP) with "click" chemistry; that is azide (azido-oligo(ethylene glycol) (OEG)-NH2) inks were printed on alkyne-terminated SAMs on silicon or SOS through PDMS stamps. The surface characterization results for the sample printed with a flat featureless PDMS stamp demonstrated a nondestructive and efficient method of μCP to perform "click" reactions on alkyne-terminated, oxide-free silicon surfaces for the first time. For the sample printed with a featured PDMS stamp, LAPS imaging showed a good agreement with the pattern of the PDMS stamp, indicating the successful chemical patterning on non-oxidized silicon and SOS substrates and the capability of LAPS to image the molecular patterns with high sensitivity.

Active Acetylcholinesterase Immobilization on a Functionalized Silicon Surface.

In this work, we studied the attachment of active acetylcholinesterase (AChE) enzyme on a silicon substrate as a potential biomarker for the detection of organophosphorous (OP) pesticides. A multistep functionalization strategy was developed on a crystalline silicon surface: a carboxylic acid-terminated monolayer was grafted onto a hydrogen-terminated silicon surface by photochemical hydrosilylation, and then AChE was covalently attached through amide bonds using an activation EDC/NHS process. Each step of the modification was quantitatively characterized by ex-situ Fourier transform infrared spectroscopy in attenuated-total-reflection geometry (ATR-FTIR) and atomic force microscopy (AFM). The kinetics of enzyme immobilization was investigated using in situ real-time infrared spectroscopy. The enzymatic activity of immobilized acetylcholinesterase enzymes was determined with a colorimetric test. The surface concentration of active AChE was estimated to be Γ = 1.72 × 10(10) cm(-2).

Scanning tunneling spectroscopy reveals a silicon dangling bond charge state transition

We report the study of single dangling bonds (DBs) on a hydrogen-terminated silicon (100) surface using a low-temperature scanning tunneling microscope. By investigating samples prepared with different annealing temperatures, we establish the critical role of subsurface arsenic dopants on the DB electronic properties. We show that when the near-surface concentration of dopants is depleted as a result of 1250 °C flash anneals, a single DB exhibits a sharp conduction step in its I(V) spectroscopy that is not due to a density of states effect but rather corresponds to a DB charge state transition. The voltage position of this transition is perfectly correlated with bias-dependent changes in the STM images of the DB at different charge states. Density functional theory calculations further highlight the role of subsurface dopants on DB properties by showing the influence of the DB-dopant distance on the DB state. We discuss possible theoretical models of electronic transport through the DB that could account for our experimental observations.

Degradation mechanism of hydrogen-terminated porous silicon in the presence and in the absence of light

Si is well-known semiconductor that has a fundamental bandgap energy of 1.12 eV. Its photogenerated electrons in the conduction band can react with the ubiquitous oxygen molecules to yield ⋅O2− radicals, but the photogenerated holes in the valance band can’t interact with OH− to produce ⋅OH radicals. In this paper, we study the degradation of methyl orange (MO) by hydrogen-terminated porous Si (H-PSi) in the presence and in the absence of light. The absorption spectra of the degraded MO solutions indicated that the H-PSi had superior degradation ability. In the dark, the reduction of dye occurs simply by hydrogen transfer. Under room light, however, some of the dye molecules can be reduced by hydrogen transfer first and then decomposed in the conduction and valance bands. This result should be ascribed to its wide band gap energies centered at 1.79-1.94 eV.

Controlling the dopant dose in silicon by mixed-monolayer doping.

Molecular monolayer doping (MLD) presents an alternative to achieve doping of silicon in a nondestructive way and holds potential for realizing ultrashallow junctions and doping of nonplanar surfaces. Here, we report the mixing of dopant-containing alkenes with alkenes that lack this functionality at various ratios to control the dopant concentration in the resulting monolayer and concomitantly the dopant dose in the silicon substrate. The mixed monolayers were grafted onto hydrogen-terminated silicon using well-established hydrosilylation chemistry. Contact angle measurements, X-ray photon spectroscopy (XPS) on the boron-containing monolayers, and Auger electron spectroscopy on the phosphorus-containing monolayers show clear trends as a function of the dopant-containing alkene concentration. Dynamic secondary-ion mass spectroscopy (D-SIMS) and Van der Pauw resistance measurements on the in-diffused samples show an effective tuning of the doping concentration in silicon.

Formation of stable Si-O-C submonolayers on hydrogen-terminated silicon(111) under low-temperature conditions.

In this letter, we report results of a hydrosilylation carried out on bifunctional molecules by using two different approaches, namely through thermal treatment and photochemical treatment through UV irradiation. Previously, our group also demonstrated that in a mixed alkyne/alcohol solution, surface coupling is biased towards the formation of Si–O–C linkages instead of Si–C linkages, thus indirectly supporting the kinetic model of hydrogen abstraction from the Si–H surface (Khung, Y. L. et al. Chem. – Eur. J. 2014, 20, 15151–15158). To further examine the probability of this kinetic model we compare the results from reactions with bifunctional alkynes carried out under thermal treatment (<130 °C) and under UV irradiation, respectively. X-ray photoelectron spectroscopy and contact angle measurements showed that under thermal conditions, the Si–H surface predominately reacts to form Si–O–C bonds from ethynylbenzyl alcohol solution while the UV photochemical route ensures that the alcohol-based alkyne may also form Si–C bonds, thus producing a monolayer of mixed linkages. The results suggested the importance of surface radicals as well as the type of terminal group as being essential towards directing the nature of surface linkage.

Charge control of silicon chemistry

Surface Chemistry A site on an organic molecule often can be made more or less reactive by changing its neighboring functional groups so that they add or withdraw electronic charge from the site. Piva et al. show a similar effect for the reaction of dangling bond states on a hydrogen-terminated silicon surface with unsaturated organic molecules such as styrene. They modified the electronic properties of the surface by changing the surface concentration of arsenic dopants and used scanning tunneling microscopy to monitor product formation. Negatively charged doubly occupied dangling bonds, which were more prevalent on the highly doped surfaces, were less reactive than neutral singly occupied dangling bonds. These results are consistent with density functional theory calculations and help explain the heterogeneous reactivity of dangling bonds on silicon surfaces. Phys. Rev. B 90 , 155422 (2014).

Enhanced photocatalytic degradation of methylene blue by metal-modified silicon nanowires

Silicon nanowires (SiNWs) modified with Au, Pt and Pd nanoparticles were used as heterogeneous photocatalysts for the photodegradation of methylene blue in water under UV light irradiation. The modification of SiNWs was carried out by deposition of metal nanoparticles using the electroless metal deposition (EMD) technique. The effect of metal nanoparticles deposition time on the photocatalytic activity was studied. It was found that the photocatalytic activity of modified SiNWs was enhanced when the deposition time of metal nanoparticles was increased. In addition of modified SiNWs with Pt, Au and Pd nanoparticles, oxidized silicon substrate (Ox-Si), oxidized silicon nanowires (Ox-SiNWs) and hydrogen-terminated silicon nanowires (H-SiNWs) were also evaluated for the photodegradation of methylene blue.