Silicon Nanoribbons Research Articles

Suppressed thermal transport in silicon nanoribbons by inhomogeneous strain.

Nanoscale structures can produce extreme strain that enables unprecedented material properties, such as tailored electronic bandgap1-5, elevated superconducting temperature6,7 and enhanced electrocatalytic activity8,9. While uniform strains are known to elicit limited effects on heat flow10-15, the impact of inhomogeneous strains has remained elusive owing to the coexistence of interfaces16-20 and defects21-23. Here we address this gap by introducing inhomogeneous strain through bending individual silicon nanoribbons on a custom-fabricated microdevice and measuring its effect on thermal transport while characterizing the strain-dependent vibrational spectra with sub-nanometre resolution. Our results show that a strain gradient of 0.112% per nanometre could lead to a drastic thermal conductivity reduction of 34 ± 5%, in clear contrast to the nearly constant values measured under uniform strains10,12,14,15. We further map the local lattice vibrational spectra using electron energy-loss spectroscopy, which reveals phonon peak shifts of several millielectron-volts along the strain gradient. This unique phonon spectra broadening effect intensifies phonon scattering and substantially impedes thermal transport, as evidenced by first-principles calculations. Our work uncovers a crucial piece of the long-standing puzzle of lattice dynamics under inhomogeneous strain, which is absent under uniform strain and eludes conventional understanding.

Printed n- and p-Channel Transistors using Silicon Nanoribbons Enduring Electrical, Thermal, and Mechanical Stress.

Printing technologies are changing the face of electronics with features such as resource-efficiency, low-cost, and novel form factors. While significant advances have been made in terms of organic electronics, the high-performance and stable transistors by printing, and their large-scale integration leading to fast integrated circuits remains a major challenge. This is because of the difficulties to print high-mobility semiconducting materials and the lack of high-resolution printing techniques. Herein, we present silicon based printed n- and p-channel transistors to demonstrate the possibility of developing high-performance complementary metal-oxide-semiconductor (CMOS) computing architecture. The direct roll transfer printing is used here for deterministic assembly of high-mobility single crystal silicon nanoribbons arrays on a flexible polyimide substrate. This is followed by high-resolution electrohydrodynamic printing to define source/drain/gate electrodes and to encapsulate, thus leading to printed devices. The printed transistors show effective peak mobilities of 15 cm2/(V s) (n-channel) and 5 cm2/(V s) (p-channel) at low 1 V drain bias. Furthermore, the effect of electrical, mechanical, and thermal stress on the performance and stability of the encapsulated transistors is investigated. The transistors showed stable transfer characteristics even after: (i) continuous 4000 transfer cycles, (ii) excruciating 10000 bending cycles at different bending radii (40, 25, and 15 mm), and (iii) between 15 and 60 °C temperatures.

First steps of silicene growth on an insulating thin-film: effect of the substrate temperature

Silicene is a two-dimensional (2D) material with very promising electronic properties for applications in silicon modern technology. However, the first experimental synthesis of silicene on metallic surfaces shows strong interactions between the silicene and its substrate, which can alter its electronic properties. Here, we report on the first steps of silicene growth on an insulating surface (NaCl) using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angle-resolved photoemission spectroscopy (ARPES). We demonstrate the importance of temperature annealing in the growth of silicene on NaCl. Indeed, after deposition of silicon on the NaCl/Ag(110) surface, we observe the following stages: (i) at room temperature, the silicon atoms accumulate on top of the NaCl layer without any given order. (ii) At 60 °C, silicon dimers start to grow on the NaCl. (iii) At 140 °C, these dimers form a 2D silicon chains on the surface. (iv) After a post-annealing at 200 °C, evident 2D silicon nanoribbons with a honeycomb-like structure were observed. Our results of the first silicene growth stages on an insulating surface are a necessary step for exploring its growth mechanism further.

High-performance p-channel transistors on flexible substrate using direct roll transfer stamping

Flexible electronics with high-performance devices are crucial for transformative advances in several emerging and traditional applications. To address this need, herein we present p-type silicon (Si) nanoribbons (NR)-based high-performance field-effect transistors (FETs) developed using an innovative direct roll transfer stamping (DRTS) process. First, ultrathin Si NRs (∼70 nm) are obtained from silicon on insulator wafers using the conventional top-down method, and then the DRTS method is employed to directly place the NRs onto flexible substrates at RT. The NRFETs are then developed following the RT fabrication process which includes deposition of high-quality SiN x dielectric. The fabricated p-channel transistors demonstrate high linear mobility ∼100 ± 10 cm2 V−1 s−1, current on/off ratio >104, and low gate leakage (<1 nA). Further, the transistors showed robust device performance under mechanical bending and at a wide temperature range (15 °C–90 °C), showing excellent potential for futuristic high-performance flexible electronic devices/circuits.

Observation of One-Dimensional Dirac Fermions in Silicon Nanoribbons.

Dirac materials, which feature Dirac cones in the reciprocal space, have been one of the hottest topics in condensed matter physics in the past decade. To date, 2D and 3D Dirac Fermions have been extensively studied, while their 1D counterparts are rare. Recently, Si nanoribbons (SiNRs), which are composed of alternating pentagonal Si rings, have attracted intensive attention. However, the electronic structure and topological properties of SiNRs are still elusive. Here, by angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy measurements, first-principles calculations, and tight-binding model analysis, we demonstrate the existence of 1D Dirac Fermions in SiNRs. Our theoretical analysis shows that the Dirac cones derive from the armchairlike Si chain in the center of the nanoribbon and can be described by the Su-Schrieffer-Heeger model. These results establish SiNRs as a platform for studying the novel physical properties in 1D Dirac materials.

Direct roll transfer of silicon nanoribbons onto polyimide substrates accomplished in one step

Direct roll transfer of silicon nanoribbons onto polyimide substrates accomplished in one step

Electronic density of states and optical spectra of zigzag SiC nanoribbons from first principles

The electronic and optical behavior of zigzag silicon carbide nanoribbons (ZSiCNRs) have been simulated exploiting density functional theory (DFT) formalism. Our results show that zigzag silicon nanoribbons (w = 14 to 20) have metallic behavior. The optical graphs confirm that ε1x(ω), ε1y(ω), nx(ω), and ny(ω) are less than that of the z polarization at energies below 2.5 eV. The static refractive index of ZSiCNRs is almost close to that of the other nanoribbons, except for the GNRs, which have a considerably larger static refractive index. Last but not least, the optical susceptibility shows that ZSiCNRs with larger width have a weaker polarization field which eventuates in lower optical susceptibility for the ZSiCNRs (w = 20).

Structure of one-dimensional monolayer Si nanoribbons on Ag(111)

One-dimensional silicon nanoribbons (SiNRs) have the potential for application to future electronic devices because of their compatibility with current silicon-based electronic devices and their theoretical outstanding electronic properties, such as the size-dependent band gap. Here we grew SiNRs by the deposition of Si on a Ag(111) surface. We investigated SiNRs on the Ag(111) surface using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. The atomic resolution STM and AFM images revealed that SiNRs have the same width and align along the equivalent orientations of Ag(111). The main body of SiNRs was found to be symmetric about the long axes of the ribbons, but the terminals of SiNRs break the symmetry. The DFT calculation results revealed a buckled single-layer structure of SiNRs, which is composed of Si hexagonal and tetragonal rings.

Optical Monitoring of the Production Quality of Si-Nanoribbon Chips Intended for the Detection of ASD-Associated Oligonucleotides.

Gas-phase etching and optical lithography were employed for the fabrication of a silicon nanoribbon chip (Si-NR chip). The quality of the so-fabricated silicon nanoribbons (Si-NRs) was monitored by optical Raman scattering spectroscopy. It was demonstrated that the structures of the Si-NRs were virtually defect-free, meaning they could be used for highly sensitive detection of biological macromolecules. The Si-NR chips were then used for the highly sensitive nanoelectronics detection of DNA oligonucleotides (oDNAs), which represent synthetic analogs of 106a-5p microRNA (miR-106a-5p), associated with the development of autism spectrum disorders in children. The specificity of the analysis was attained by the sensitization of the Si-NR chip sur-face by covalent immobilization of oDNA probes, whose nucleotide sequence was complementary to the known sequence of miR-106a-5p. The use of the Si-NR chip was demonstrated to al-low for the rapid label-free real-time detection of oDNA at ultra-low (~10−17 M) concentrations.

Giant photothermoelectric effect in silicon nanoribbon photodetectors

The photothermoelectric (PTE) effect enables efficient harvesting of the energy of photogenerated hot carriers and is a promising choice for high-efficiency photoelectric energy conversion and photodetection. Recently, the PTE effect was reported in low-dimensional nanomaterials, suggesting the possibility of optimizing their energy conversion efficiency. Unfortunately, the PTE effect becomes extremely inefficient in low-dimensional nanomaterials, owing to intrinsic disadvantages, such as low optical absorption and immature fabrication methods. In this study, a giant PTE effect was observed in lightly doped p-type silicon nanoribbons caused by photogenerated hot carriers. The open-circuit photovoltage responsivity of the device was 3-4 orders of magnitude higher than those of previously reported PTE devices. The measured photovoltage responses fit very well with the proposed photothermoelectric multiphysics models. This research proposes an application of the PTE effect and a possible method for utilizing hot carriers in semiconductors to significantly improve their photoelectric conversion efficiency.

Kink as a new degree of freedom to tune the thermal conductivity of Si nanoribbons

An attractive feature of nanomaterials is the possibility of tuning their properties through controlling their size and surface morphology, and understanding the effects of various parameters on thermal transport properties of nanostructures has been an active research topic in the past two decades. Through systematic studies of kinked silicon nanoribbons, we show how the kink morphology, a newly recognized degree of freedom for tuning thermal transport in nanostructures, modulates the thermal conductivity of these nanoribbons. For kinked Si nanoribbons that are 34 nm thick and 141 nm wide, the measured thermal conductivity first decreases as the period length reduces from 2 μm to 0.5 μm, reaching a 21% thermal conductivity reduction as compared to that of a straight counterpart at 300 K. However, as the period length drops to a level at which a straight heat transfer channel opens between the heat source and the sink, the thermal conductivity exhibits a steep increasing trend. Moreover, the comparison of thermal conductivity reduction for kinked ribbons along different crystalline directions indicates that phonon focusing could be exploited to further suppress thermal transport in kinked silicon nanoribbons. These results provide important guidelines on modulating heat transfer in nanostructures using kinks, which could be adopted to tune the thermal properties of nanostructures for different applications, such as thermoelectrics, microelectronic device thermal management, and functional thermal regulators.

How kinks can tune the thermal conductivity of silicon nanoribbons

Experimental and numerical studies reveal the underlying mechanisms of how kink morphology modulates the thermal conductivity of nanoribbons.

Wrinkling of silicon nanoribbons on shape memory polymers

Shape memory polymers (SMPs) have recently emerged as the substrate for surface wrinkling with potential applications in smart adhesion and tunable optical gratings. In this paper, wrinkling of single-crystal silicon nanoribbons on epoxy SMP substrate is investigated experimentally and theoretically. With the unique shape fixing capability of SMPs, the well-defined wavy profile of silicon nanoribbons is obtained. A simple theoretical mechanics model, which bypasses the complex thermomechanical properties of SMPs, based on the existing wrinkling mechanics is proposed to explain the underlying physics behind the wrinkled silicon nanoribbons. Theoretical predictions on wavy profile agree very well with experiments. It is found that the wavy profile is independent of the recovery temperature, which is the high temperature TH above the transition temperature Tg. The material properties of SMPs at the transition temperature play a key role in determining the wavy profile. These results provide a scientific basis to design SMP-supported surface morphologies with important applications in smart surface engineering.

Thermal transport through fishbone silicon nanoribbons: unraveling the role of Sharvin resistance.

Heat conduction has been shown to be greatly suppressed in Si nanomeshes, which has attracted extensive attention for potential thermoelectric applications, yet the precise suppression mechanism remains to be fully understood. Attempting to further disclose the underlying mechanisms, we report on the thermal conductivity of the building block for nanomeshes, i.e., Si nanoribbons with fins attached to the two opposite sides. By expanding only the fin width while keeping both the period length and the backbone size constant, we observed an unexpected non-monotonic trend of the effective thermal conductivity normalized with the backbone cross-section. Further analysis showed that the corrected thermal conductivity extracted with appropriate consideration of the geometrical effect on diffusion followed a monotonically decreasing trend, reaching a maximum thermal conductivity reduction of 18% at 300 K for a ribbon with the maximum explored fin width of 430 nm, as compared to that of the straight ribbon of 66 nm backbone width. We attribute the thermal conductivity reduction to the thermal constriction resistance induced by the cross-section reduction between the fin and backbone sections. For ribbons with a larger fin width, the effective phonon mean free path is longer for phonons arriving at the constriction, which boosts the ballistic constriction resistance, i.e., Sharvin resistance, and leads to a lower thermal conductivity.

Unoccupied electronic band structure of pentagonal Si nanoribbons on Ag(110).

Silicon nanoribbons - one dimensional silicon structures with a pentagonal atomic structure and mixed sp2- and sp3-hybridisation - grow on Ag(110) upon deposition of silicon. These nanostructures are viewed as promising candidates for modern day electronics as they are comprised of the same element as today's semiconductor devices. Even though they have been studied extensively over the last decade, only little is known about their unoccupied band structure which is important for possible future optoelectronics, semiconductor, and spintronics applications. In order to elucidate the unoccupied band structure of the nanoribbons, k-resolved inverse photoemission spectroscopy (KRIPES) studies were performed on both nanoribbon structures reported in the literature as well as on the bare Ag(110) substrate within the energy range of E-EF = 0-6.5 eV. The obtained experimental results are compared to density functional theory (DFT) calculated band structures to assign individual spectral features to specific bands. Since even small changes in the structural model of the nanoribbons lead to a change in the calculated band structure, this comparison allows us to assess the validity of the proposed structural models.

Structure determination of substrate influenced silicon nano-ribbon growth

Using photoelectron diffraction (XPD), photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), we investigate the structure of silicon nano-ribbons grown on Au(1 1 0). We find that silicon forms two zigzag-terminated hexagonal chains along the [1‾ 1 0]-substrate direction, while the gold substrate forms a bulk-typed surface reconstruction after silicon evaporation. Only two missing rows along the [1‾ 1 0]-direction remain partially in their original structure at the Si/Au interface. Unlike silicon nano-ribbon formation on Ag(1 1 0), nano-ribbon growth on Au(1 1 0) is strongly substrate influenced by Van-der-Waals interaction and by AuSi bonding formation. This strong interaction leads to a substrate-driven hexagon formation, where the gold atoms are partially integrated into the ribbon structure. Besides the determination of the internal silicon nano-ribbon structure, we identify the origin of the two chemically shifted silicon components in the XPS spectra by deconvolving the diffraction pattern.

Sensing the electrical activity of single ion channels with top-down silicon nanoribbons

Using top-down fabricated silicon nanoribbons, we measure the opening and closing of ion channels alamethicin and gramicidin A. A capacitive model of the system is proposed to demonstrate that the geometric capacitance of the nanoribbon is charged by ion channel currents. The integration of top-down nanoribbons with electrophysiology holds promise for integration of electrically active living systems with artificial electronics.

Covalent functionalization of polycrystalline silicon nanoribbons applied to Pb(II) electrical detection

Although recent studies in the field of nanomaterials have allowed the realization of highly sensitive sensors, research regarding new types of functionalization of nanostructures to reach high selectivity are still missing. Here, a simple electronic device based on polycrystalline silicon (poly-Si) nanoribbons used for lead detection is presented. This device is functionalized by reduction of aryl diazonium salts in order to detect heavy metals. The effectiveness of the grafting process and thickness homogeneity of the coating layer were evaluated by XPS and NanoSIMS techniques. The preconcentration of lead (Pb2+) at the surface of the functionalized nanostructures was substantially verified. Finally, electrical characterization of the resistors based on the functionalized nanoribbons, showed that the sensitivity to these species is increased in the concentration ranging from 10−7 to 10−5 mol L−1. This technique could pave the way for use of complexing agents to enhance heavy metal detection.

The Pentagonal Nature of Self-Assembled Silicon Chains and Magic Clusters on Ag(110)

The atomic structures of self-assembled silicon nanoribbons and magic clusters on Ag(110) substrate have been studied by high-resolution noncontact atomic force microscopy (nc-AFM) and tip-enhanced Raman spectroscopy (TERS). Pentagon-ring structures in Si nanoribbons and clusters have been directly visualized. Moreover, the vibrational fingerprints of individual Si nanoribbon and cluster retrieved by subnanometer resolution TERS confirm the pentagonal nature of both Si nanoribbons and clusters. This work demonstrates that Si pentagon can be an important element in building silicon nanostructures, which may find important applications for future nanoelectronic devices based on silicon.

First Principles Study on the Magnetism of Rectangular Nanosilicenes**Supported by the National Natural Science Foundation of China under Grant Nos 61376102 and 11174048, and the Computational Support from Shanghai Supercomputer Center.

We present first-principle calculations on the magnetism in finite rectangular nanosilicenes (RNSs). An antiferromagnetic (AFM) state at two zigzag edges is found when the RNSs approach a critical size. This AFM state originates from the localized pz orbits of Si atoms at the edges, similar to those in the infinitely long zigzag-edged silicon nanoribbons. The smallest RNS that can maintain the AFM phase as the ground state is identified. It is also found that aluminum dopants can regulate the distribution of the spin density and the energy difference between AFM and FM states.