Roie Yerushalmi

Stimuli responsive materials : new avenues toward smart organic devices

“Smart” patternable polymer-based materials that can be designed from various molecular building blocks show great potential, as they may be used in many fields, including nanotechnology, biochemistry, organic and physical chemistry, and materials science. The focus of this highlight will be on the basic design characteristics of practical Stimuli Responsive Materials (SRMs), the wide range of potential applications and the challenges to be accomplished in this rapidly expanding area. In particular, recent developments are described which are related to two of the many fundamental aspects of stimuli triggered responses: those that are photo-triggered and those that are solvent triggered. These selected state-of-the-art examples demonstrate the large scope and diversity in terms of activation mechanism, response time and property control.

Wafer-scale, sub-5 nm junction formation by monolayer doping and conventional spike annealing.

We report the formation of sub-5 nm ultrashallow junctions in 4 in. Si wafers enabled by the molecular monolayer doping of phosphorus and boron atoms and the use of conventional spike annealing. The junctions are characterized by secondary ion mass spectrometry and noncontact sheet resistance measurements. It is found that the majority ( approximately 70%) of the incorporated dopants are electrically active, therefore enabling a low sheet resistance for a given dopant areal dose. The wafer-scale uniformity is investigated and found to be limited by the temperature homogeneity of the spike anneal tool used in the experiments. Notably, minimal junction leakage currents (<1 microA/cm(2)) are observed that highlights the quality of the junctions formed by this process. The results clearly demonstrate the versatility and potency of the monolayer doping approach for enabling controlled, molecular-scale ultrashallow junction formation without introducing defects in the semiconductor.

Large scale, highly ordered assembly of nanowire parallel arrays by differential roll printing

A differential roll printing strategy is developed to enable large-scale and uniform assembly of highly aligned and ordered nanowire arrays on various rigid and flexible substrate materials. The dynamics of the process are explored by tuning the linear sliding motion of the roller with respect to the rolling motion, clearly demonstrating the importance of the differential rolling process in the controlled assembly of nanowires. The potency and versatility of the method is further demonstrated by fabrication of nanowire transistor arrays on flexible substrates.

Contact Doping of Silicon Wafers and Nanostructures with Phosphine Oxide Monolayers

Contact doping method for the controlled surface doping of silicon wafers and nanometer scale structures is presented. The method, monolayer contact doping (MLCD), utilizes the formation of a dopant-containing monolayer on a donor substrate that is brought to contact and annealed with the interface or structure intended for doping. A unique feature of the MLCD method is that the monolayer used for doping is formed on a separate substrate (termed donor substrate), which is distinct from the interface intended for doping (termed acceptor substrate). The doping process is controlled by anneal conditions, details of the interface, and molecular precursor used for the formation of the dopant-containing monolayer. The MLCD process does not involve formation and removal of SiO(2) capping layer, allowing utilization of surface chemistry details for tuning and simplifying the doping process. Surface contact doping of intrinsic Si wafers (i-Si) and intrinsic silicon nanowires (i-SiNWs) is demonstrated and characterized. Nanowire devices were formed using the i-SiNW channel and contact doped using the MLCD process, yielding highly doped SiNWs. Kelvin probe force microscopy (KPFM) was used to measure the longitudinal dopant distribution of the SiNWs and demonstrated highly uniform distribution in comparison with in situ doped wires. The MLCD process was studied for i-Si substrates with native oxide and H-terminated surface for three types of phosphorus-containing molecules. Sheet resistance measurements reveal the dependency of the doping process on the details of the surface chemistry used and relation to the different chemical environments of the P═O group. Characterization of the thermal decomposition of several monolayer types formed on SiO(2) nanoparticles (NPs) using TGA and XPS provides insight regarding the role of phosphorus surface chemistry at the SiO(2) interface in the overall MLCD process. The new MLCD process presented here for controlled surface doping provides a simple yet highly versatile means for achieving postgrowth doping of nanometer scale structures and interfaces.

Transformation of Organic–Inorganic Hybrid Films Obtained by Molecular Layer Deposition to Photocatalytic Layers with Enhanced Activity

We present the transformation of organic-inorganic hybrid titanicone films formed by TiCl(4) as metal precursor and ethylene glycol (EG) using solvent-free MLD to highly active photocatalytic films. The photocatalytic activities of the films were investigated using hydroxyl-functionalized porphyrin as a spectroscopic marker. TEM imaging and electron diffraction, XPS, UV-vis spectroscopy, and spectroscsopic ellipsometry were employed for structural and composition analyses of the films. The photocatalytic activity of Ti-EG films was investigated for different anneal temperatures and compared to TiO(2) films prepared by ALD using TiCl(4) as metal precursor and H(2)O (TiO(2) films). Overall, our results indicate that the photocatalytic activity of the thermally annealed Ti-EG film is about 5-fold increased compared to that of the TiO(2) film prepared by ALD for optimal process conditions. The combined results indicate that the structural and photocatalytic properties can be assigned to three states: (I) amorphous state, intermediate dye loading, low photocatalytic activity, (II) intermediate film state with both crystalline and amorphous regions, high dye loading, high catalytic activity, and (III) highly crystalline film with low dye loading and low photocatalytic activity. The formation of photocatalytic nanotubes (NTs) is demonstrated using sacrificial Ge nanowires (NWs) scaffolds to yield Ti-EG NT structures with controllable wall thickness structures and enhanced dye loading capacity. Our results demonstrate the feasibility and high potential of MLD to form metal oxides with high photocatalytic activity.

Sustainable photocatalytic production of hydrogen peroxide from water and molecular oxygen

We report the direct production of H2O2 from O2 and H2O using a heterogeneous catalyst made from non-toxic materials using light as the sole energy source for the process. Photocatalytic production of H2O2 is demonstrated using light energy without the need for additional chemical energy (sacrificial compounds) or applied electrical potential. Fine-tuning of catalyst architecture and interface design enables exceptional photocatalytic activity.

Phosphine Oxide Monolayers on SiO2 Surfaces

Molecular thin films utilizing phosphonates are widely used in a variety of chemical applications, such as surfactants, stabilization of nanoparticle suspensions, layer by layer film formation, and more. Furthermore, surface chemistry is a valuable tool in the context of semiconducting materials. To date, the studies of monolayer formation of phosphonates have mainly focused on phosphonic acids. However, for certain applications, the less polar phosphine oxides are more desirable. Phosphine oxides are readily soluble in nonpolar organic solvents, such as toluene and mesitylene, and are compatible with many types of semiconducting materials. Importantly, these phosphorous derivatives do not have acidic functionality which can lead to uncontrolled etching or degradation of the substrate surfaces. Recently, we demonstrated a novel application using phosphine oxides for controlled nanoscale doping of materials by utilization of their surface chemical properties. Herein, we report the details of monolayer formation of phosphine oxides on SiO2 substrates, shedding light on the critical role of their chemical substituents in the assembly process. We find the monolayer formation process to be self-limiting, without applying specialized techniques as required for phosphonic acid thin films. Furthermore, the precise nature of the binding interactions between the phosphine oxides and the SiO2 substrate, and the uniformity and surface morphology of the self-assembled monolayers, are found to strongly depend on the chemical, electronic, and steric properties of the substituents. The chemical binding interactions between the phosphine oxides and SiO2 surfaces may involve packing interactions, hydrogen bonding, and/or covalent bonding, depending on the chemical substituents (Figure 1b). Such interactions can lead to monolayer formation involving the P=O molecular sites. Monolayer formation for precursors 1 and 2 was confirmed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), whereas no surface reaction was observed for 3 (Figure 2). H-bonds and covalent bonds are expected to be energetically less favorable for 3 than for 1 and 2 because of the more electron-withdrawing substituents of the P=O group as compared to 1 and 2. This Figure 1. a) The molecular precursors used in this study, and b) proposed H-bond and covalent-bond monolayer formation on the SiO2 surfaces.

Parallel p–n Junctions across Nanowires by One-Step Ex Situ Doping

The bottom-up synthesis of nanoscale building blocks is a versatile approach for the formation of a vast array of materials with controlled structures and compositions. This approach is one of the main driving forces for the immense progress in materials science and nanotechnology witnessed over the past few decades. Despite the overwhelming advances in the bottom-up synthesis of nanoscale building blocks and the fine control of accessible compositions and structures, certain aspects are still lacking. In particular, the transformation of symmetric nanostructures to asymmetric nanostructures by highly controlled processes while preserving the modified structural orientation still poses a significant challenge. We present a one-step ex situ doping process for the transformation of undoped silicon nanowires (i-Si NWs) to p-type/n-type (p-n) parallel p-n junction configuration across NWs. The vertical p-n junctions were measured by scanning tunneling microscopy (STM) in concert with scanning tunneling spectroscopy (STS), termed STM/S, to obtain the spatial electronic properties of the junction formed across the NWs. Additionally, the parallel p-n junction configuration was characterized by off-axis electron holography in a transmission electron microscope to provide an independent verification of junction formation. The doping process was simulated to elucidate the doping mechanisms involved in the one-step p-i-n junction formation.

Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex Situ Doping: A Correlative Study via Atom-Probe Tomography and Scanning Tunneling Spectroscopy

Dopants play a critical role in modulating the electric properties of semiconducting materials, ranging from bulk to nanoscale semiconductors, nanowires, and quantum dots. The application of traditional doping methods developed for bulk materials involves additional considerations for nanoscale semiconductors because of the influence of surfaces and stochastic fluctuations, which may become significant at the nanometer-scale level. Monolayer doping is an ex situ doping method that permits the post growth doping of nanowires. Herein, using atom-probe tomography (APT) with subnanometer spatial resolution and atomic-ppm detection limit, we study the distributions of boron and phosphorus in ex situ doped silicon nanowires with accurate control. A highly phosphorus doped outer region and a uniformly boron doped interior are observed, which are not predicted by criteria based on bulk silicon. These phenomena are explained by fast interfacial diffusion of phosphorus and enhanced bulk diffusion of boron, respectively. The APT results are compared with scanning tunneling spectroscopy data, which yields information concerning the electrically active dopants. Overall, comparing the information obtained by the two methods permits us to evaluate the diffusivities of each different dopant type at the nanowire oxide, interface, and core regions. The combined data sets permit us to evaluate the electrical activation and compensation of the dopants in different regions of the nanowires and understand the details that lead to the sharp p-i-n junctions formed across the nanowire for the ex situ doping process.

Facile Monolayer Formation on SiO2 Surfaces via Organoboron Functionalities

More than they appear on the surface: The treatment of SiO2 nanoparticles under mild conditions with two organoboron derivatives led to boron-containing monolayers with different types of surface species (see picture) through the direct formation of Si-O-B bonds. The organoboron-modified SiO2 NPs showed selective reactivity towards diols.