[Show abstract][Hide abstract] ABSTRACT: Patterned fabrication depends on selective deposition that can be best achieved with atomic layer deposition (ALD). For the growth of TiO2 by ALD using TiCl4 and H2O, X-ray photoelectron spectroscopy reveals a marked difference in growth on oxidized and hydrogen-terminated silicon surfaces, characterized by typical and predictable deposition rates observed on SiO2 surfaces that can be 185 times greater than the deposition rates on hydrogen-terminated Si(100) and Si(111) surfaces. Large-scale patterning is demonstrated using wet chemistry, and nanometer-scale patterned TiO2 growth is achieved through scanning tunneling microscopy (STM) tip-based lithography and ALD. The initial adsorption mechanisms of TiCl4 on clean, hydrogen-terminated, and OH-terminated Si(100)-(2 × 1) surfaces are investigated in detail through density functional theory calculations. Varying the reactive groups on the substrate is found to strongly affect the probability of precursor nucleation on the surface during the ALD process. Theoretical studies provide quantitative understanding of the experimental differences obtained for the SiO2, hydrogen-terminated, and clean Si(100) and Si(111) surfaces.
No preview · Article · Oct 2013 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: We use time-resolved photoluminescence (PL) kinetics and PL intensity measurements to study the decay of photoexcitations in colloidal CdSe/ZnS nanocrystals grafted on SiO2−Si substrates with a wide range of the SiO2 spacer layer thicknesses. The salient features of experimental observations are found to be in good agreement with theoretical expectations within the framework of modification of spontaneous decay of electric-dipole excitons by their environment. Analysis of the experimental data reveals that energy transfer (ET) from nanocrystals into Si is a major enabler of substantial variations in decay rates, where we quantitatively distinguish contributions from nonradiative and radiative ET channels. We demonstrate that time-resolved PL kinetics provides a more direct assessment of ET, while PL intensity measurements are also affected by the specifics of the generation and emission processes.
No preview · Article · Sep 2013 · Journal of the Optical Society of America B
[Show abstract][Hide abstract] ABSTRACT: Semiconductor manufacturing has provided a fabrication technology with ever-improving precision that is extremely cost-effective at producing transistors but provides poor relative precision at the limits of its resolution. Recent efforts at developing tip based nanofabrication are breaking out of 2D into 3D structures that achieve atomic precision with top down control. Coincidentally, applications that require atomic precision have begun to emerge. This talk will review progress towards atomically precise manufacturing, describe some of the near term applications such as nano electro mechanical systems, sensors, QuBit devices, and nanoimprint templates, and prospects for scaling up the throughput of atomically precise fabrication technology.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Two different organic monolayers were prepared on silicon Si(111) and modified for attaching gold nanoparticles. The molecules are covalently bound to silicon and form very ordered monolayers sometimes improperly called Self-Assembled Monolayers (SAM). They are designed to be electrically insulating and to have very few electrical interface states. By positioning the tip of an STM above a nanoparticle a Double Barrier Tunnel Junction (DBTJ) is created and Coulomb blockade is demonstrated at 40K. This is the first time Coulomb blockade is observed with an organic monolayer on oxide-free silicon. This work focuses on the fabrication and initial electrical characterization of this double barrier tunnel junction. The organic layers were prepared by thermal hydrosilylation of two different alkene molecules with either a long carbon chain (C11) or a shorter one (C7) and both were modified to be amine-terminated. FTIR and XPS measurements confirm that the Si(111) substrate remains unoxidized during the whole chemical process. Colloidal gold nanoparticles were prepared using two methods: either with citrate molecules (Turkevich method) or with ascorbic acid as the surfactant. In both cases AFM and STM images show a well-controlled deposition on the grafted organic monolayer. I-V curves obtained by Scanning Tunneling Spectroscopy (STS) are presented on 8 nm diameter nanoparticles and exhibit the well-known Coulomb staircases at low temperature. The curves are discussed as a function of the organic layer thickness and silicon substrate doping.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t Exfoliated graphite nanoplatelet (xGnP) and V 2 O 5 nanotube (VNT) composite electrodes were fabri-cated and tested as electrodes for supercapacitors. Enhancement of the electrochemical performance of the composite over its component materials was demonstrated using cyclic voltammetry (CV), con-stant current charge/discharge testing and electrochemical impedance spectroscopy in aqueous and organic electrolytes. The calculated specific capacitance of the composite was 35 F g −1 in the aqueous electrolyte and 226 F g −1 in the organic electrolyte at a scan rate of 10 mV s −1 using a three-electrode sys-tem. Asymmetric coin cell devices were fabricated using activated carbon cloth as the positive electrode and xGnP–VNT composite as the negative electrode. The energy and power densities in 1 M LiTFSI were 28 Wh kg −1 and 303 W kg −1 , respectively, at a discharge current density of 250 mA g −1 . Published by Elsevier B.V.
[Show abstract][Hide abstract] ABSTRACT: Nanostructured materials attract great interest as candidates for
producing new, practical photoelectronic devices. Many current devices
are based on charge-transfer in which primary photoexcitations are
separated into an electron and hole on different sides of the interface.
Poor interface quality and carrier transport are issues that result in a
lower conversion efficiencies than in inorganic crystalline devices. An
alternative is given by non-radiative energy transfer (NRET) based
hybrid nanostructures, which combine strongly absorbing components such
as nanocrystal quantum dots (NQDs) and high-mobility semiconductor
layers. In this work, we compared the effectiveness of 1,6-hexanedithiol
vs. 1,6-hexanediamine to link multilayer NQD structures. Steady state
photoluminescence (PL) measurements showed that using 1,6-hexanediamine
consistently resulted in higher PL counts and passivation of the NQDs.
Furthermore, we studied bilayer structures of different size NQD layers
(565NQDs on 605NQDs) linked with 1,6-hexanediamine. We performed
time-resolved and steady-state PL measurements to quantify the NRET
rates between the 565NQD layer and the 605NQD layer. NRET rates were
consistently 91%. Hence, we foresee the utilization of bilayer NQD
structures linked with 1,6-hexanediamine in energy transfer-based
[Show abstract][Hide abstract] ABSTRACT: Silicon is by far the most important semiconductor material in the microelectronic industry mostly due to the high quality of the Si/SiO2 interface. Consequently, applications requiring chemical functionalization of Si substrates have focused on molecular grafting of SiO2 surfaces. Unfortunately, there are practical problems affecting homogeneity and stability of many organic layers grafted on SiO2, such as silanes and phosphonates, related to polymerization and hydrolysis of Si–O–Si and Si–O–P bonds. These issues have stimulated efforts in grafting functional molecules on oxide-free Si surfaces, mostly with wet chemical processes. This review focuses therefore directly on wet chemical surface functionalization of oxide-free Si surfaces, starting from H-terminated Si surfaces. The main preparation methods of oxide-free H-terminated Si and their stability are first summarized. Functionalization is then classified into indirect substitution of H-termination by functional organic molecules, such as hydrosilylation, and direct substitution by other atoms (e.g. halogens) or small functional groups (e.g. OH, NH2) that can be used for further reaction. An emphasis is placed on a recently discovered method to produce a nanopattern of functional groups on otherwise oxide-free, H-terminated and atomically flat Si(1 1 1) surfaces. Such model surfaces are particularly interesting because they make it possible to derive fundamental knowledge of surface chemical reactions.
[Show abstract][Hide abstract] ABSTRACT: The intrinsic electrical properties of thin organic layers/semiconductor interfaces are challenging to probe because of large tunneling currents through the molecular layer and damage during device fabrication. We present here a method to protect the interface with an Al2O3 layer gently deposited on top using atomic layer deposition. The resulting two-layered gate stack can be characterized with capacitance and conductance measurements. The protected interface shows an inherent high quality with low Dit, less than 2 × 1011 cm−2 eV−1 for n-type Si(111) surface, best measured using mercury probe. The additional procedures required to fabricate MOS capacitors lead to an increase in Dit.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate efficient excitonic sensitization of crystalline Si nanomembranes via combined effects of radiative (RET) and nonradiative (NRET) energy transfer from a proximal monolayer of colloidal semiconductor nanocrystals. Ultrathin, 25-300 nm Si films are prepared on top of insulating SiO(2) substrates and grafted with a monolayer of CdSe/ZnS nanocrystals via 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. Time-resolved photoluminescence measurements combined with theoretical modeling allow us to quantify individual contributions from RET and NRET. Overall efficiency of ET into Si is estimated to exceed 85% for a short distance of about 4 nm from nanocrystals to the Si surface. Effective and longer-range radiative coupling of nanocrystal's emission to waveguiding modes of Si films is clearly revealed. This demonstration supports the feasibility of an advanced thin-film hybrid solar cell concept that relies on energy transfer between strong light absorbers and adjacent high-mobility Si layers.
[Show abstract][Hide abstract] ABSTRACT: In this study, TiO2 nanotube (TNT)/reduced graphene oxide (hGO) composites were prepared by an alkaline hydrothermal process. This was achieved by decorating graphene oxide (GO) layers with commercially available TiO2 nanoparticles (P90) followed by hydrothermal synthesis, which converts the TiO2 nanoparticles to small diameter (9 nm) TNTs on the hGO surface. The alkaline medium used to synthesize the TNTs simultaneously converts GO to deoxygenated graphene oxide (hGO). Compared to GO, the hGO has a 70% reduction of oxygenated species after alkaline hydrothermal treatment. The graphene nature of hGO in the composites was confirmed by X-ray diffraction (XRD), Raman, FTIR, and X-ray photoelectron spectroscopy (XPS) analysis. The photocatalytic performance of the hGO-TNT composites was evaluated for the photodegradation of malachite green. It was found that the ratio of hGO to TNT in the composites significantly affects the photocatalytic activity. Higher amounts of hGO in hGO-TNT composites showed lower photocatalytic activity than pure TNTs. The composite with 10% hGO showed the highest photocatalytic activity, with a 3-fold enhancement in photocatalytic efficiency over pure TNTs. It is expected that the synthesis of “high surface area-small diameter” TiO2 nanotubes and simultaneous conversion of GO to graphene like hGO “without using strong reducing agents” could be a promising strategy for preparing other types of carbon based TiO2 nanotube composite photocatalysts.
[Show abstract][Hide abstract] ABSTRACT: Exfoliated graphite nanoplatelet (xGnP) and V2O5 nanotube (VNT) composite electrodes were fabricated and tested as electrodes for supercapacitors. Enhancement of the electrochemical performance of the composite over its component materials was demonstrated using cyclic voltammetry (CV), constant current charge/discharge testing and electrochemical impedance spectroscopy in aqueous and organic electrolytes. The calculated specific capacitance of the composite was 35Fg−1 in the aqueous electrolyte and 226Fg−1 in the organic electrolyte at a scan rate of 10mVs−1 using a three-electrode system. Asymmetric coin cell devices were fabricated using activated carbon cloth as the positive electrode and xGnP–VNT composite as the negative electrode. The energy and power densities in 1M LiTFSI were 28Whkg−1 and 303Wkg−1, respectively, at a discharge current density of 250mAg−1.
No preview · Article · Apr 2012 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: Silicon-on-insulator (SOI) field-effect-based sensors are often biased using a back substrate gate with biological/electrolytic solutions placed over the top sensor channel. For electrically floating electrolytes in contact with the SOI-buried oxide, we demonstrate that the electrolyte voltage is capacitively coupled to the applied back-gate bias resulting in a dual-gated sensor. Measured electrolyte voltages approach the constant back-gate voltage for real-time measurements, while a slower response is observed with dynamic measurements. Low subthreshold swings and threshold voltages are observed in both experimental and simulatedI(d)-V-g curves, consistent with strong coupling. Real-time constant voltage sensing can thus be performed at much lower operating voltages.
No preview · Article · Mar 2012 · IEEE Electron Device Letters
[Show abstract][Hide abstract] ABSTRACT: Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to 80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron–phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids.
[Show abstract][Hide abstract] ABSTRACT: This paper describes the development of a high vacuum fracture rig for delaminating functionalized silicon surfaces. The main focus here is on examining molecular interactions by functionalizing the silicon surfaces with carboxyl and diamine terminated self-assembled monolayers (SAMs). However, many other interactions can be considered. The crack front location and normal crack opening displacements (NCOD) are measured by infrared crack opening interferometry (IR-COI). This allows the normal traction-separation relation of the interactions to be determined. Some mixed-mode fracture experiments were conducted on silicon/carboxyl/diamine/silicon sandwich specimens in ambient and high vacuum. Interesting differences in behavior were noted.
No preview · Article · Feb 2012 · Experimental Mechanics
[Show abstract][Hide abstract] ABSTRACT: To optimize colloidal nanocrystals/Si hybrid structures, nanopillars are prepared and organized via microparticle patterning and Si etching. A monolayer of CdSe nanocrystals is then grafted on the passivated oxide-free nanopillar surfaces, functionalized with carboxy-alkyl chain linkers. This process results to a negligible number of non-radiative surface state defects with a tightly controlled separation between the nanocrystals and Si. Steady-state and time-resolved photoluminescence measurements confirm the close-packing nanocrystal arrangement and the dominance of non-radiative energy transfer from nanocrystals to Si. We suggest that radially doped p-n junction devices based on energy transfer offer a viable approach for thin film photovoltaic devices.
No preview · Article · Jan 2012 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: XeF2 treatment of aluminum and alumina surfaces is known to produce hydrophilic surfaces. There is however poor knowledge of the chemical nature of these surfaces. Using infrared absorption and X-ray photoelectron spectroscopy, the formation of highly hydrophilic AlF3 and AlOxFy surface layers is identified upon XeF2 exposure, along with strongly bound H2O and other related surface species formed by interactions with trace H2O under typical vacuum conditions (≈10–4 Torr). Surfaces resulting from XeF2 etching of oxide-free aluminum covered by a sacrificial Si layer have a strong affinity for H2O, with a contact-angle of ca. 5–10°. First-principles simulations offer new insight into details of the AlFx surface structure, based on the surface IR characterization by providing reliable assignments for associated AlFx···H2O infrared bands and showing that fluorine is strongly bound to Al, preventing further Al oxidation. The formation of hydrophilic AlF3 surface layers upon fluorine-based etching may pose a fundamental limitation for the use of Al in microelectromechanical (MEMs) applications, precluding the release of low-stiction, low-capillary force components.
Full-text · Article · Oct 2011 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Vanadium pentoxide (V2O5) layered nanostructures are known to have very stable crystal structures and high faradaic activity. The low electronic conductivity of V2O5 greatly limits the application of vanadium oxide as electrode materials and requires combining with conducting materials using binders. It is well known that the organic binders can degrade the overall performance of electrode materials and need carefully controlled compositions. In this study, we develop a simple method for preparing freestanding carbon nanotube (CNT)-V2O5 nanowire (VNW) composite paper electrodes without using binders. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode (Spectracarb 2225) as the cathode. The supercapacitor with CNT-VNW composite paper electrode exhibits a power density of 5.26 kW Kg−1 and an energy density of 46.3 Wh Kg−1. (Li)VNWs and CNT composite paper electrodes can be fabricated in similar manner and show improved overall performance with a power density of 8.32 kW Kg−1 and an energy density of 65.9 Wh Kg−1. The power and energy density values suggest that such flexible hybrid nanocomposite paper electrodes may be useful for high performance electrochemical supercapacitors.
No preview · Article · Oct 2011 · Advanced Energy Materials
[Show abstract][Hide abstract] ABSTRACT: The potential and electric field boundary conditions for the Gouy-Chapman model of the electrolyte diffuse layer are used to properly couple the potentials in the silicon-on-insulator-based metal-oxide-semiconductor field-effect transistor to the electrolyte. This analysis is possible because the active silicon channel is fully depleted. Both the subthreshold and linear regimes are simulated. An operation with electrolyte floating and bias applied to the substrate is compared with the other methods of biasing the sensor.
No preview · Article · Jul 2011 · IEEE Transactions on Electron Devices