What a difference a bond makes: the structural, chemical, and physical properties of methyl-terminated Si(111) surfaces

Electrochemical Impedance of Well-Passivated Semiconductors Reveals Bandgaps, Fermi Levels, and Interfacial Density of States

For well-passivated semiconductor materials, the density of states (DOS) at the band edge determines the concentration of electrons (or holes) available to participate in photo/electrochemical redox and chemical reactions. Electrochemical impedance enables the characterization of photo-electrode DOS in a functional, in situ, electrochemical environment. However, the in situ electrochemical approach remains underutilized for band structure characterization of inorganic semiconductors. In this work, we demonstrate that the DOS of the well-passivated, highly ordered semiconductors silicon and germanium is directly probed by electrochemical impedance spectroscopy (EIS). More specifically, EIS measurements of the chemical capacitance in contact with electrolyte enable direct analysis of the DOS properties. From the capacitance-potential plot, the following parameters can be extracted: Fermi level, valence band maximum, conduction band minimum, and a quantitative value of the number of states at each potential. This study aims to establish the groundwork for future EIS investigations of electronically modified semiconductor interfaces with covalently bound organic molecules, organometallic catalysts, or more complex biorelated functionalizations.

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Methylation of Si(111) Modulates Molecular Orientation in Perylenediimide Thin Films

Singlet fission produces a pair of low-energy spin-triplet excitons from a single high-energy spin-singlet exciton. While this process offers the potential to enhance the efficiency of silicon solar cells by ∼30%, meeting this goal requires overlayer materials that can efficiently transport triplet excitons to an underlying silicon substrate. Herein, we demonstrate that the chemical functionalization of silicon surfaces controls the structure of vapor-deposited thin films of perylenediimide (PDI) dyes, which are prototypical singlet fission materials. Using a combination of atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we find terminating Si(111) with either a thin, polar oxide layer (SiOx) or with hydrophobic methyl groups (Si-CH3) alters the structures of the resulting PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an "edge-on" orientation with respect to the silicon surface, films grown on Si-CH3 contain large grains that prefer to align in a "face-on" manner with respect to the substrate. This "face-on" orientation is expected to enhance exciton transport to silicon. Interestingly, we find that the preferred mode of growth for different PDIs correlates with the space group associated with bulk crystals of these compounds. While PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse crystalline columns, PDIs that inhabit triclinic (P1̅) space groups afford films comprised of uniform, lamellar PDI domains. The results highlight that silicon surface functionalization profoundly impacts PDI thin film growth, and rational selection of a hydrophobic surface that promotes "face-on" adsorption may improve energy transfer to silicon.

Silicon - single molecule - silicon circuits

In 2020, silicon - molecule - silicon junctions were fabricated and shown to be on average one third as conductive as traditional junctions made using gold electrodes, but in some instances to be even more conductive, and significantly 3 times more extendable and 5 times more mechanically stable. Herein, calculations are performed of single-molecule junction structure and conductivity pertaining to blinking and scanning-tunnelling-microscopy (STM) break junction (STMBJ) experiments performed using chemisorbed 1,6-hexanedithiol linkers. Some strikingly different characteristics are found compared to analogous junctions formed using the metals which, to date, have dominated the field of molecular electronics. In the STMBJ experiment, following retraction of the STM tip after collision with the substrate, unterminated silicon surface dangling bonds are predicted to remain after reaction of the fresh tips with the dithiol solute. These dangling bonds occupy the silicon band gap and are predicted to facilitate extraordinary single-molecule conductivity. Enhanced junction extendibility is attributed to junction flexibility and the translation of adsorbed molecules between silicon dangling bonds. The calculations investigate a range of junction atomic-structural models using density-functional-theory (DFT) calculations of structure, often explored at 300 K using molecular dynamics (MD) simulations. These are aided by DFT calculations of barriers for passivation reactions of the dangling bonds. Thermally averaged conductivities are then evaluated using non-equilibrium Green's function (NEGF) methods. Countless applications through electronics, nanotechnology, photonics, and sensing are envisaged for this technology.

Silicon Surface Passivation for Silicon-Colloidal Quantum Dot Heterojunction Photodetectors

Sensitizing crystalline silicon (c-Si) with an infrared-sensitive material, such as lead sulfide (PbS) colloidal quantum dots (CQDs), provides a straightforward strategy for enhancing the infrared-light sensitivity of a Si-based photodetector. However, it remains challenging to construct a high-efficiency photodetector based upon a Si:CQD heterojunction. Herein, we demonstrate that Si surface passivation is crucial for building a high-performance Si:CQD heterojunction photodetector. We have studied one-step methyl iodine (CH₃I) and two-step chlorination/methylation processes for Si surface passivation. Transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements reveal that the two-step passivated Si:CQD interface exhibits fewer trap states and decreased recombination rates. These passivated substrates were incorporated into prototype Si:CQD infrared photodiodes, and the best performance photodiode based upon the two-step passivation shows an external quantum efficiency (EQE) of 31% at 1280 nm, which represents a near 2-fold increase over the standard device based upon the one-step CH₃I passivated Si.

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study

We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using ab initio molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N₂ from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs. The nonadiabatic couplings necessary for a description of the excited-state dynamics are calculated by combining ab initio electronic structures and reduced density matrix formalism with Redfield theory. The phenyl-terminated slab shows reduced nonradiative relaxation and recombination rates of hot charge carriers in comparison with the hydrogen-terminated slab. Altogether, our results provide atomistic insights revealing that (i) the diazonium salt thermally decomposes at the surface allowing the formation of covalently bonded phenyl group, and (ii) the coverage of phenyl groups on the surface slows down charge carrier cooling driven by electron-phonon interactions, which increases photoluminescence efficiency at the near-infrared spectral region.

In Situ Analytical Techniques for the Investigation of Material Stability and Interface Dynamics in Electrocatalytic and Photoelectrochemical Applications

Electrocatalysis and photoelectrochemistry are critical to technologies like fuel cells, electrolysis, and solar fuels. Material stability and interfacial phenomena are central to the performance and long-term viability of these technologies. Researchers need tools to uncover the fundamental processes occurring at the electrode/electrolyte interface. Numerous analytical instruments are well-developed for material characterization, but many are ex situ techniques often performed under vacuum and without applied bias. Such measurements miss dynamic phenomena in the electrolyte under operational conditions. However, innovative advancements have allowed modification of these techniques for in situ characterization in liquid environments at electrochemically relevant conditions. This review explains some of the main in situ electrochemical characterization techniques, briefly explaining the principle of operation and highlighting key work in applying the method to investigate material stability and interfacial properties for electrocatalysts and photoelectrodes. Covered methods include spectroscopy (in situ UV-vis, ambient pressure X-ray photoelectron spectroscopy (APXPS), and in situ Raman), mass spectrometry (on-line inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectrometry (DEMS)), and microscopy (in situ transmission electron microscopy (TEM), electrochemical atomic force microscopy (EC-AFM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical microscopy (SECM)). Each technique's capabilities and advantages/disadvantages are discussed and summarized for comparison.

Effect of Covalent Surface Functionalization of Si on the Activity of Trifluoromethanesulfonic Anhydride for Suppressing Surface Recombination

An examination of the efficacy of combining physisorbed and chemisorbed passivation strategies on crystalline Si has been performed. This report compares the influence of a linear alkyl adsorbate tethered by either a Si-C or Si-Si linkage, prepared by reaction of Si(111) with organometallic Grignard reagents or organosilanes, respectively. These modified surfaces are first analyzed and compared by IR and X-ray photoelectron spectroscopies. Their behavior toward a known potent physisorbate, trifluoromethanesulfonic anhydride (Tf₂O), is then examined. Microwave photoconductivity measurements were obtained which indicate that, while Tf₂O shows a beneficial lowering of surface recombination on both surface types initially, only surfaces featuring Si-C linkages exhibit long-lasting suppressed surface recombination. The data for Grignard-treated Si after exposure to Tf₂O in fact represent the longest known report of surface recombination suppression by a physisorbate. Conversely, the data for the Si surfaces prepared by dehydrogenative coupling suggest that these passivating groups themselves introduce defect states that cannot be ameliorated by Tf₂O physisorption.

Surface chemistry effects on work function, ionization potential and electronic affinity of Si(100), Ge(100) surfaces and SiGe heterostructures

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (-I, -Br, -Cl, -F) and other chemical functionalizations (-H, -OH, -CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar -OH and -CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases.

Response of Photoluminescence of H-Terminated and Hydrosilylated Porous Si Powders to Rinsing and Temperature

The photoluminescence (PL) response of porous Si has potential applications in a number of sensor and bioimaging techniques. However, many questions still remain regarding how to stabilize and enhance the PL signal, as well as how PL responds to environmental factors. Regenerative electroless etching (ReEtching) was used to produce photoluminescent porous Si directly from Si powder. As etched, the material was H-terminated. The intensity and peak wavelength were greatly affected by the rinsing protocol employed. The highest intensity and bluest PL were obtained when dilute HCl(aq) rinsing was followed by pentane wetting and vacuum oven drying. Roughly half of the hydrogen coverage was replaced with –RCOOH groups by thermal hydrosilylation. Hydrosilylated porous Si exhibited greater stability in aqueous solutions than H-terminated porous Si. Pickling of hydrosilylated porous Si in phosphate buffer was used to increase the PL intensity without significantly shifting the PL wavelength. PL intensity, wavelength and peak shape responded linearly with temperature change in a manner that was specific to the surface termination, which could facilitate the use of these parameters in a differential sensor scheme that exploits the inherent inhomogeneities of porous Si PL response.

Spontaneous S-Si bonding of alkanethiols to Si(111)-H: towards Si-molecule-Si circuits

We report the synthesis of covalently linked self-assembled monolayers (SAMs) on silicon surfaces, using mild conditions, in a way that is compatible with silicon-electronics fabrication technologies. In molecular electronics, SAMs of functional molecules tethered to gold via sulfur linkages dominate, but these devices are not robust in design and not amenable to scalable manufacture. Whereas covalent bonding to silicon has long been recognized as an attractive alternative, only formation processes involving high temperature and/or pressure, strong chemicals, or irradiation are known. To make molecular devices on silicon under mild conditions with properties reminiscent of Au–S ones, we exploit the susceptibility of thiols to oxidation by dissolved O₂, initiating free-radical polymerization mechanisms without causing oxidative damage to the surface. Without thiols present, dissolved O₂ would normally oxidize the silicon and hence reaction conditions such as these have been strenuously avoided in the past. The surface coverage on Si(111)–H is measured to be very high, 75% of a full monolayer, with density-functional theory calculations used to profile spontaneous reaction mechanisms. The impact of the Si–S chemistry in single-molecule electronics is demonstrated using STM-junction approaches by forming Si–hexanedithiol–Si junctions. Si–S contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 2.7 s, which is five folds higher than that reported for conventional molecular junctions formed between gold electrodes. The enhanced “ON” lifetime of this single-molecule circuit enables previously inaccessible electrical measurements on single molecules.

Spontaneously formed Si–S bonds enable monolayer and single-molecule Si–molecule–Si circuits.

Modification of a H-terminated silicon surface by organic sulfide molecules: the mechanism and origin of reactivity

For the reactions of disulfide molecules (RSSR), the steric effect rather than the electronic effect of the R group is the main origin of the different reactivity. In the reactions of sulfide molecules (RSXR′, X = S, P, Si, O, N, C), charges on the S atom and dissociation energies of the S–X bonds have a great impact on the reactivity of these reactions.

Vibrational Sum Frequency Generation Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces

The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120° jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 °C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 ± 360 cm^-1 and an attempt frequency of (2.9 ± 4.2) × 10¹³ s^-1 for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising carbon-free approach to produce hydrogen from water. A photoelectrochemical cell consists of a semiconductor photoelectrode in contact with an aqueous electrolyte. Its performance is sensitive to properties of the photoelectrode/electrolyte interface, which may be tuned through functionalization of the photoelectrode surface with organic molecules. This can lead to improvements in the photoelectrode's properties. This Minireview summarizes key computational investigations on using molecular functionalization to modify photoelectrode stability, barrier height, and catalytic activity. It is discussed how first-principles density functional theory, first-principles molecular dynamics, and device modeling simulations can provide predictive insights and complement experimental investigations of functionalized photoelectrodes. Challenges and future directions in the computational modeling of functionalized photoelectrode/electrolyte interfaces within the context of experimental studies are also highlighted.

A Mini Review: Recent Advances in Surface Modification of Porous Silicon

Porous silicon has been utilized within a wide spectrum of industries, as well as being used in basic research for engineering and biomedical fields. Recently, surface modification methods have been constantly coming under the spotlight, mostly in regard to maximizing its purpose of use. Within this review, we will introduce porous silicon, the experimentation preparatory methods, the properties of the surface of porous silicon, and both more conventional as well as newly developed surface modification methods that have assisted in attempting to overcome the many drawbacks we see in the existing methods. The main aim of this review is to highlight and give useful insight into improving the properties of porous silicon, and create a focused description of the surface modification methods.

1,2-Epoxyalkane: Another Precursor for Fabricating Alkoxy Self-Assembled Monolayers on Hydrogen-Terminated Si(111)

This work describes the UV alkoxylation of a series of 1,2-epoxyalkanes on the hydrogen-terminated silicon (H-Si) substrate. The formation of alkoxy self-assembled monolayers (SAMs) and the nature of bonding at the surface of H-Si were examined using water contact angle goniometer, spectroscopic ellipsometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy. UV exposure to 1,2-epoxyalkane mesitylene solution for 60 min formed alkoxy-SAMs onto H-Si with hydrophobic properties. The local molecular environment of the alkyl chains transitioned from a disordered, liquid-like state to an ordered, crystalline-like structure with increasing the chain length. XPS and FTIR indicated that the reaction of H-Si with 1,2-epoxyalkane produced Si-O-C linkages. The Si-H bond homolysis and electron/hole were the plausible mechanistic routes for the grafting of 1,2-epoxyalkanes.

Synthetic Insights into Surface Functionalization of Si(111)-R Photoelectrodes: Steric Control and Deprotection of Molecular Passivating Layers

We report the utility of controlled spacing of molecular monolayers on Si(111) surfaces by the use of sterically bulky silanes. The steric bulk of a 3,5-diphenolic linker of type Ph-diO-SiR₃ (R = hexyl, phenyl, ⁱPr)-as well as the smaller Ph-diOMe-is shown to control the surface coverage on Si(111). The para substituent was also changed from -F (small) to -OTf (triflate, large) to modulate the conformation of a selected bulky silane (SiR₃; R = hexyl) to further control the steric environment of the monolayer. The surface coverage values are found to vary systematically from 57 → 21 → 15 → 11% for the series CH₃ → hexyl → ⁱPr → phenyl. Substitution at the para position (F → OTf) decreased the packing density for R = hexyl to as low as 8% (from 21%). The molecular coverage was also found to control the rate and extent of surface oxidation when unfunctionalized sites were allowed to oxidize. Following attachment, facile deprotection of the silanes was achieved by treatment with BBr₃ to afford the diphenolic -OH groups. To electronically characterize the monolayers, voltammetry was performed in contact with liquid Hg to determine the barrier height, which was decreased by 70 mV as the coverage is increased. This study provides a synthetic rationale for controlling the packing density of surface linkers using electroless chemistry at semiconductor interfaces, thus providing further tunability and functionality of photoelectrochemical devices.

High-Density Modification of H-Terminated Si(111) Surfaces Using Short-Chain Alkynes

H-Si(111)-terminated surfaces were alkenylated via two routes: through a novel one-step gas-phase hydrosilylation reaction with short alkynes (C₃ to C₆) and for comparison via a two-step chlorination and Grignard alkenylation process. All modified surfaces were characterized by static water contact angles and X-ray photoelectron spectroscopy (XPS). Propenyl- and butenyl-coated Si(111) surfaces display a significantly higher packing density than conventional C₁₀-C₁₈ alkyne-derived monolayers, showing the potential of this approach. In addition, propyne chemisorption proceeds via either of two approaches: the standard hydrosilylation at the terminal carbon (lin) at temperatures above 90 °C and an unprecedented reaction at the second carbon (iso) at temperatures below 90 °C. Molecular modeling revealed that the packing energy of a monolayer bonded at the second carbon is significantly more favorable, which drives iso-attachment, with a dense packing of surface-bound iso-propenyl chains at 40% surface coverage, in line with the experiments at <90 °C. The highest density monolayers are obtained at 130 °C and show a linear attachment of 1-propenyl chains with 92% surface coverage.

Synthesis and Characterization of Alkylamine-Functionalized Si(111) for Perovskite Adhesion With Minimal Interfacial Oxidation or Electronic Defects

We investigated synthetic strategies for the functionalization of Si(111) surfaces with organic species containing amine moieties. We employed the functionalized surfaces to chemically "glue" perovskites to silicon with efficient electron transfer and minimal oxidation leading to deleterious recombination at the silicon substrate. A two-step halogenation-alkylation reaction produced a mixed allyl-methyl monolayer on Si(111). Subsequent reactions utilized multiple methods of brominating the allyl double bond including reaction with HBr in acetic acid, HBr in THF, and molecular bromine in dichloromethane. Reaction with ammonia in methanol effected conversion of the bromide to the amine. X-ray photoelectron spectroscopy (XPS) quantified chemical states and coverages, transient-microwave photoconductivity ascertained photogenerated carrier lifetimes, atomic force microscopy (AFM) quantified perovskite-silicon adhesion, and nonaqueous photoelectrochemistry explored solar-energy-conversion performance. The HBr bromination followed by the amination yielded a surface with ∼10% amine sites on the Si(111) with minimal oxide and surface recombination velocity values below 120 cm s^-1, following extended exposures to air. Importantly, conversion of amine sites to ammonium and deposition of methylammonium lead halide via spin coating and annealing did not degrade carrier lifetimes. AFM experiments quantified adhesion between perovskite films and alkylammonium-functionalized or native-oxide silicon surfaces. Adhesion forces/interactions between the perovskite and the alkylammonium-functionalized films were comparable to the interaction between the perovskite and native-oxide silicon surface. Photoelectrochemistry of perovskite thin films on alkylammonium-functionalized n⁺-Si showed significantly higher V_oc than n⁺-Si with a native oxide when in contact with a nonaqueous ferrocene^+/0 redox couple. We discuss the present results in the context of utilizing molecular organic recognition to attach perovskites to silicon utilizing organic linkers so as to inexpensively modify silicon for future tandem-junction photovoltaics.

Thermal Stability of Organic Monolayers Grafted to Si(111): Insights from ReaxFF Reactive Molecular Dynamics Simulations

We used the ReaxFF reactive molecular dynamics simulations to investigate the chemical mechanisms and kinetics of thermal decomposition processes of silicon surfaces grafted with different organic molecules via Si-C bonds at atomistic level. In this work, we considered the Si(111) surface grafted with n-alkyl (ethyl, propyl, pentyl, and decyl) layers in 50% coverage and, Si-CH₃, Si-CCCH₃ and Si-CHCHCH₃ layers in full coverage. Si radicals primarily formed by the homolytic cleavage of Si-C bonds play a key role in the dehydrogenation processes that lead to the decomposition of the monolayers. Contrary to commonly proposed mechanisms that only involve a single Si atom center, we found that the main decomposition pathways require two Si lattice atoms to proceed. The ability of surface silyl radicals to dehydrogenate the organic molecules depends on the flexibility of the carbon backbones of the organic molecules as well as on the C-H bond strength. The dehydrogenation of n-alkyl chains mainly involves the H atoms of the β-carbon (leading to 1-alkene desorption). However, as the surface coverage decreases, the flexibility of the alkyl chains allows for the dehydrogenation of any methylene group and even the terminal methyl group of the long decyl layer. On the contrary, the rigid carbon backbone of the Si-CCCH₃ and Si-CHCHCH₃ moieties hinders the dehydrogenation of the terminal methyl group, which confers these layers a higher thermal stability. For all layers, the surface ends up mostly hydrogenated as Si-C bonds break and new Si-H bonds are formed during the dehydrogenation reactions.

Rapid Surface Functionalization of Hydrogen-Terminated Silicon by Alkyl Silanols

Surface functionalization of inorganic semiconductor substrates, particularly silicon, has focused attention toward many technologically important applications, involving photovoltaic energy, biosensing and catalysis. For such modification processes, oxide-free (H-terminated) silicon surfaces are highly required, and different chemical approaches have been described in the past decades. However, their reactivity is often poor, requiring long reaction times (2–18 h) or the use of UV light (10–30 min). Here, we report a simple and rapid surface functionalization for H-terminated Si(111) surfaces using alkyl silanols. This catalyst-free surface reaction is fast (15 min at room temperature) and can be accelerated with UV light irradiation, reducing the reaction time to 1–2 min. This grafting procedure leads to densely packed organic monolayers that are hydrolytically stable (even up to 30 days at pH 3 or 11) and can display excellent antifouling behavior against a range of organic polymers.

Chemical Modification of Semiconductor Surfaces for Molecular Electronics

Inserting molecular monolayers within metal/semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones. The review covers the main aspects of using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts. One of the greatest merits of molecular monolayers is their capability to form excellent thin dielectrics, yielding rich and unique current-voltage characteristics for transport across metal/molecular monolayer/semiconductor interfaces. We explain the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region, on the other hand, as well as how different monolayer chemistries control each of these barriers. Practical tools to experimentally identify these two barriers and distinguish between them are given, followed by a short look to the future. This review is accompanied by another one, concerning the formation of large-area molecular junctions and charge transport that is dominated solely by molecules.

Half-flat vs. atomically flat: Alkyl monolayers on morphologically controlled Si(100) and Si(111) have very similar structure, density, and chemical stability

Chemists have long preferred the Si(111) surface for chemical functionalization, as a simple aqueous etch can be used to produce ideal, atomically flat H/Si(111) surfaces for subsequent reactions. In contrast, industry-standard etches produce rough H/Si(100) surfaces terminated by nanohillocks. The recent discovery of an aqueous etch that produces morphologically controlled H/Si(100) surfaces with a near atomically flat or "half-flat" morphology challenges the assumption that Si(111) is an inherently preferable starting point for chemical functionalization. This study shows that alkyl functionalization of morphologically controlled, "half-flat" Si(100) surfaces by terminal alkenes produces dense, tightly packed monolayers that are essentially identical to those on atomically flat Si(111). The striking similarity between the infrared spectra on these two surfaces - in terms of absolute absorbance, line shape and position, and polarization dependence - strongly suggests that alkyl monolayers on morphologically controlled Si(111) and Si(100) have essentially identical structures. The principle difference between the two surfaces is the amount of residual H at the Si/organic interface, a difference that is dictated by the structure of the Si(100) surface. Alkyl monolayers on morphologically controlled Si(111) and Si(100) surfaces were shown to be equally resistant to harsh oxidants. As a result, there appears to be no chemical reason to prefer one surface over the other, at least for functionalization with terminal alkenes.

Experimental and theoretical study of rotationally inelastic diffraction of H2(D2) from methyl-terminated Si(111)

Fundamental details concerning the interaction between H2 and CH3-Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H2 and D2 from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H2 compared to the strong RID features observed for D2 over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH3-Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H2 and D2 have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H2 (D2) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H2(D2)/CH3-Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H2 with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems.

Binary functionalization of H:Si(111) surfaces by alkyl monolayers with different linker atoms enhances monolayer stability and packing

Alkyl monolayer modified Si forms a class of inorganic-organic hybrid materials with applications across many technologies such as thin-films, fuel/solar-cells and biosensors. Previous studies have shown that the linker atom, through which the monolayer binds to the Si substrate, and any tail group in the alkyl chain, can tune the monolayer stability and electronic properties. In this paper we study the H:Si(111) surface functionalized with binary SAMs: these are composed of alkyl chains that are linked to the surface by two different linker groups. Aiming to enhance SAM stability and increase coverage over singly functionalized Si, we examine with density functional theory simulations that incorporate vdW interactions, a range of linker groups which we denote as -X-(alkyl) with X = CH2, O(H), S(H) or NH(2) (alkyl = C6 and C12 chains). We show how the stability of the SAM can be enhanced by adsorbing alkyl chains with two different linkers, e.g. Si-[C, NH]-alkyl, through which the adsorption energy is increased compared to functionalization with the individual -X-alkyl chains. Our results show that it is possible to improve stability and optimum coverage of alkyl functionalized SAMs linked through a direct Si-C bond by incorporating alkyl chains linked to Si through a different linker group, while preserving the interface electronic structure that determines key electronic properties. This is important since any enhancement in stability and coverage to give more densely packed monolayers will result in fewer defects. We also show that the work function can be tuned within the interval of 3.65-4.94 eV (4.55 eV for bare H:Si(111)).

Expanding the Repertoire of Molecular Linkages to Silicon: Si-S, Si-Se, and Si-Te Bonds

Silicon is the foundation of the electronics industry and is now the basis for a myriad of new hybrid electronics applications, including sensing, silicon nanoparticle-based imaging and light emission, photonics, and applications in solar fuels, among others. From interfacing of biological materials to molecular electronics, the nature of the chemical bond plays important roles in electrical transport and can have profound effects on the electronics of the underlying silicon itself, affecting its work function, among other things. This work describes the chemistry to produce ≡Si-E bonds (E = S, Se, and Te) through very fast microwave heating (10-15 s) and direct thermal heating (hot plate, 2 min) through the reaction of hydrogen-terminated silicon surfaces with dialkyl or diaryl dichalcogenides. The chemistry produces surface-bound ≡Si-SR, ≡Si-SeR, and ≡Si-TeR groups. Although the interfacing of molecules through ≡Si-SR and ≡Si-SeR bonds is known, to the best of our knowledge, the heavier chalcogenide variant, ≡Si-TeR, has not been described previously. The identity of the surface groups was determined by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and depth profiling with time-of-flight-secondary ionization mass spectrometry (ToF-SIMS). Possible mechanisms are outlined, and the most likely, based upon parallels with well-established molecular literature, involve surface silyl radicals or dangling bonds that react with either the alkyl or aryl dichalcogenide directly, REER, or its homolysis product, the alkyl or aryl chalcogenyl radical, RE· (where E = S, Se, and Te).

Electric and Photoelectric Properties of 3,4-Ethylenedioxythiophene-Functionalized n-Si/PEDOT:PSS Junctions

Organic/inorganic solid-state junctions play a critical role in tandem artificial photosynthetic devices supported by conducting polymer membranes. Recent work with n-Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (

PEDOT

PSS) hybrid junctions has shown that the electrical behavior is dominated by the passivating groups present on the silicon surface. In this work, the photovoltaic behavior of n-Si/

PEDOT

PSS was investigated with methyl, thiophene, and 3,4-ethylenedioxythiophene (EDOT) groups covalently attached to the Si(111) surface. X-ray photoelectron spectroscopy results demonstrated that complete monolayer coverage was achieved in 3 h and that the organic passivating groups were retained over two months of exposure to ambient conditions with minimal silicon oxidation. All surfaces investigated exhibited similar light-limited photocurrents and bulk-limited open-circuit voltages, and thiophene produced a dramatic reduction of the fill factor attributed to the formation of trap states at the interface. Furthermore, shunt behavior observed near the power-producing regions for the thiophene and EDOT surfaces is indicative of increased recombination events under forward bias and suggests that hole transport across the interface is enhanced. Thus, thiophene- and EDOT-functionalized Si(111) offer similar stabilities and efficiencies to those of methylated surfaces as well as enhanced hole transport to the

PEDOT

PSS interface from the n-Si surfaces.

Influence of steric hindrance on the molecular packing and the anchoring of quinonoid zwitterions on gold surfaces

The N-substituent on quinonoid zwitterions influences the molecules packing and impacts their anchoring on gold surfaces.

Electrical Characteristics of the Junction between PEDOT:PSS and Thiophene-Functionalized Silicon Microwires

Thiophene moieties have been attached to Si microwires (Si MWs) by a two-step chlorination/alkylation reaction method. X-ray photoelectron spectroscopy indicated that saturation of the surface occurred after 30 min of reaction time. Electrical measurements using a standard probe station indicated that the junction between individual thiophene-functionalized Si MWs and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (

PEDOT

PSS) became more ohmic as more thiophene was added to the MW surface. Under a light-limited current of 20 nA, representative of operation of Si MWs under 1 Sun illumination conditions, the iR loss of thiophene-n-Si MW/PEDOT-PSS contacts was 20 mV, representing an order of magnitude reduction compared with PEDOT-PSS junctions formed with methyl terminated n-Si MWs. Such iR losses are much less than typical catalytic overpotentials for fuel formation, and hence the thiophene-functionalized Si MW contacts will not limit the performance of a Si MW array-based solar fuels device under 1 Sun illumination.

Reduced Graphene Oxide Bipolar Membranes for Integrated Solar Water Splitting in Optimal pH

The integration of light absorbers and catalysts for the water splitting process requires a membrane capable of both ion and electron management and product separation to realize efficient solar fuels systems. Bipolar membranes can maintain a pH gradient for optimal reaction conditions by the dissociation of water. Such membranes that contain graphene in the interfacial layer are fabricated by the chemical reduction of a uniformly deposited graphene oxide layer to convert sp(3) catalyst regions to sp(2) conductive regions. The resulting electrical and water dissociation properties are optimized by adjusting the exposure conditions, and treatments of less than 5 min render an interface that exceeds the conductivity requirements for integrated solar water splitting and increases the overpotential by <0.3 V. Integration with photoelectrodes is examined by characterizing the electrical interface formed between graphene and Si microwires, and we found that efficient Ohmic junctions are possible.

Nanoscale plasmonic stamp lithography on silicon

Nanoscale lithography on silicon is of interest for applications ranging from computer chip design to tissue interfacing. Block copolymer-based self-assembly, also called directed self-assembly (DSA) within the semiconductor industry, can produce a variety of complex nanopatterns on silicon, but these polymeric films typically require transformation into functional materials. Here we demonstrate how gold nanopatterns, produced via block copolymer self-assembly, can be incorporated into an optically transparent flexible PDMS stamp, termed a plasmonic stamp, and used to directly functionalize silicon surfaces on a sub-100 nm scale. We propose that the high intensity electric fields that result from the localized surface plasmons of the gold nanoparticles in the plasmonic stamps upon illumination with low intensity green light, lead to generation of electron-hole pairs in the silicon that drive spatially localized hydrosilylation. This approach demonstrates how localized surface plasmons can be used to enable functionalization of technologically relevant surfaces with nanoscale control.

Photoelectrochemical operation of a surface-bound, nickel-phosphine H2 evolution catalyst on p-Si(111): a molecular semiconductor|catalyst construct

A molecular DuBois-type H₂ catalyst (Ni–PNP) has been covalently attached to a p-Si(111) photocathode as a molecular semiconductor|catalyst construct.