Ambipolar electrical transport in semiconducting single-wall carbon nanotubes

Black Phosphorus Field-Effect Transistors with Improved Contact via Localized Joule Heating

Two-dimensional (2D) black phosphorus (BP) is considered an ideal building block for field-effect transistors (FETs) owing to its unique structure and intriguing properties. To achieve high-performance BP-FETs, it is essential to establish a reliable and low-resistance contact between the BP and the electrodes. In this study, we employed a localized Joule heating method to improve the contact between the 2D BP and gold electrodes, resulting in enhanced BP-FET performance. Upon applying a sufficiently large source-drain voltage, the zero-bias conductance of the device increased by approximately five orders of magnitude, and the linearity of the current-voltage curves was also enhanced. This contact improvement can be attributed to the formation of gold phosphide at the interface of the BP and the gold electrodes owing to current-generated localized Joule heat. The fabricated BP-FET demonstrated a high on/off ratio of 4850 and an on-state conductance per unit channel width of 1.25 μS μm-1, significantly surpassing those of the BP-FETs without electrical annealing. These findings offer a method to achieve a low-resistance BP/metal contact for developing high-performance BP-based electronic devices.

The Schottky barrier transistor in emerging electronic devices

This paper explores how the Schottky barrier (SB) transistor is used in a variety of applications and material systems. A discussion of SB formation, current transport processes, and an overview of modeling are first considered. Three discussions follow, which detail the role of SB transistors in high performance, ubiquitous and cryogenic electronics. For high performance computing, the SB typically needs to be minimized to achieve optimal performance and we explore the methods adopted in carbon nanotube technology and two-dimensional electronics. On the contrary for ubiquitous electronics, the SB can be used advantageously in source-gated transistors and reconfigurable field-effect transistors (FETs) for sensors, neuromorphic hardware and security applications. Similarly, judicious use of an SB can be an asset for applications involving Josephson junction FETs.

Buried graphene heterostructures for electrostatic doping of low-dimensional materials

The fabrication and characterization of steep slope transistor devices based on low-dimensional materials requires precise electrostatic doping profiles with steep spatial gradients in order to maintain maximum control over the channel. In this proof-of-concept study we present a versatile graphene heterostructure platform with three buried individually addressable gate electrodes. The platform is based on a vertical stack of embedded titanium and graphene separated by an intermediate oxide to provide an almost planar surface. We demonstrate the functionality and advantages of the platform by exploring transfer and output characteristics at different temperatures of carbon nanotube field-effect transistors with different electrostatic doping configurations. Furthermore, we back up the concept with finite element simulations to investigate the surface potential. The presented heterostructure is an ideal platform for analysis of electrostatic doping of low-dimensional materials for novel low-power transistor devices.

Combining n-MOS Charge Sensing with p-MOS Silicon Hole Double Quantum Dots in a CMOS platform

Holes in silicon quantum dots are receiving attention due to their potential as fast, tunable, and scalable qubits in semiconductor quantum circuits. Despite this, challenges remain in this material system including difficulties using charge sensing to determine the number of holes in a quantum dot, and in controlling the coupling between adjacent quantum dots. We address these problems by fabricating an ambipolar complementary metal-oxide-semiconductor (CMOS) device using multilayer palladium gates. The device consists of an electron charge sensor adjacent to a hole double quantum dot. We demonstrate control of the spin state via electric dipole spin resonance. We achieve smooth control of the interdot coupling rate over 1 order of magnitude and use the charge sensor to perform spin-to-charge conversion to measure the hole singlet-triplet relaxation time of 11 μs for a known hole occupation. These results provide a path toward improving the quality and controllability of hole spin-qubits.

Carbon-Related Materials: Graphene and Carbon Nanotubes in Semiconductor Applications and Design

As the scaling technology in the silicon-based semiconductor industry is approaching physical limits, it is necessary to search for proper materials to be utilized as alternatives for nanoscale devices and technologies. On the other hand, carbon-related nanomaterials have attracted so much attention from a vast variety of research and industry groups due to the outstanding electrical, optical, mechanical and thermal characteristics. Such materials have been used in a variety of devices in microelectronics. In particular, graphene and carbon nanotubes are extraordinarily favorable substances in the literature. Hence, investigation of carbon-related nanomaterials and nanostructures in different ranges of applications in science, technology and engineering is mandatory. This paper reviews the basics, advantages, drawbacks and investigates the recent progress and advances of such materials in micro and nanoelectronics, optoelectronics and biotechnology.

Functionalization of Pristine, Metallic, and Semiconducting-SWCNTs by ZnO for Efficient Charge Carrier Transfer: Analysis through Critical Coagulation Concentration

Noncovalent functionalization of single-walled carbon nanotubes (SWCNT) by semiconducting oxides is a majorly sought technique to retain individual properties while creating a synergetic effect for an efficient heterostructure charge transfer. Three types of electronically and optically different SWCNTs: metallic (m), semiconducting (s), and pristine (p) are functionalized by ZnO using a facile sonication method. The physicochemical and morphological properties of the ZnO-functionalized SWCNTs, m-SWCNT+ZnO, s-SWCNT+ZnO, and p-SWCNT+ZnO, are analyzed by advanced characterization techniques. Evidence of charge transfer between SWCNT and ZnO is observed with an increase in charge carrier lifetime from 3.31 ns (ZnO) to 4.76 ns (s-SWCNT+ZnO). To investigate the optimum interaction between SWCNTs and ZnO, critical coagulation concentrations (CCC) are determined using UV-vis absorption spectroscopy for m-SWCNT, s-SWCNT, and p-SWCNT using different molar concentrations of ZnO as the coagulant. The interaction and coagulation mechanisms are described by the modified DLVO theory. Due to the variation in dielectric values and electronic properties of SWCNTs, the CCC values obtained have differed: m-SWCNT (1.9 × 10^-4), s-SWCNT (3.4 × 10^-4), and p-SWCNT (2 × 10^-4). An additional analysis of the aggregates and supernatants of the CCC experiments is also shown to give an insight into the interaction and coagulation processes, explaining the absence of influence exerted by sedimentation and centrifugation.

Advances and Frontiers in Single-Walled Carbon Nanotube Electronics

Single-walled carbon nanotubes (SWCNTs) have been considered as one of the most promising electronic materials for the next-generation electronics in the more Moore era. Sub-10 nm SWCNT-field effect transistors (FETs) have been realized with several performances exceeding those of Si-based FETs at the same feature size. Several industrial initiatives have attempted to implement SWCNT electronics in integrated circuit (IC) chips. Here, the recent advances in SWCNT electronics are reviewed from in-depth understanding of the fundamental electronic structures, the carrier transport mechanisms, and the metal/SWCNT contact properties. In particular, the subthreshold switching properties are highlighted for low-power, energy-efficient device operations. State-of-the-art low-power SWCNT-based electronics and the key strategies to realize low-voltage and low-power operations are outlined. Finally, the essential challenges and prospects from the material preparation, device fabrication, and large-scale ICs integration for future SWCNT-based electronics are foregrounded.

Ta2 Ni3 Se8 : 1D van der Waals Material with Ambipolar Behavior

In this study, high-purity and centimeter-scale bulk Ta₂ Ni₃ Se₈ crystals are obtained by controlling the growth temperature and stoichiometric ratio between tantalum, nickel, and selenium. It is demonstrated that the bulk Ta₂ Ni₃ Se₈ crystals could be effectively exfoliated into a few chain-scale nanowires through simple mechanical exfoliation and liquid-phase exfoliation. Also, the calculation of electronic band structures confirms that Ta₂ Ni₃ Se₈ is a semiconducting material with a small bandgap. A field-effect transistor is successfully fabricated on the mechanically exfoliated Ta₂ Ni₃ Se₈ nanowires. Transport measurements at room temperature reveal that Ta₂ Ni₃ Se₈ nanowires exhibit ambipolar semiconducting behavior with maximum mobilities of 20.3 and 3.52 cm² V^-1 s^-1 for electrons and holes, respectively. The temperature-dependent transport measurement (from 90 to 295 K) confirms the carrier transport mechanism of Ta₂ Ni₃ Se₈ nanowires. Based on these characteristics, the obtained 1D vdW material is expected to be a potential candidate for additional 1D materials as channel materials.

Ambipolar 2D Semiconductors and Emerging Device Applications

With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, light-emitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrated optoelectronic circuits, and artificial neural network image sensors, enriching the functions of conventional devices and bringing breakthroughs to build new architectures. This review first focuses on the basic knowledge including fundamental principle of ambipolar semiconductors, basic material preparation techniques, and how to obtain the ambipolar behavior through electrical contact engineering. Then, the current ambipolar 2D semiconductors and their preparation approaches and main properties are summarized. Finally, the emerging new device structures are overviewed in detail, along with their novel electronic and optoelectronic applications. It is expected to shed light on the future development of ambipolar 2D semiconductors, exploring more new devices with novel functions and promoting the applications of 2D materials.

Strengthened Complementary Metal-Oxide-Semiconductor Logic for Small-Band-Gap Semiconductor-Based High-Performance and Low-Power Application

Silicon-based complementary metal-oxide-semiconductor (CMOS) has been the mainstream logic style for modern digital integrated circuits (ICs) for decades but will meet its performance limits soon. Extensive investigations have thus been carried out using other semiconductors, especially those with extremely high carrier mobility. However, these materials usually have small or even zero band gap, which leads inevitably to large leakage current or voltage loss in ICs based on these semiconductors. In this work, we propose and demonstrate a strengthened CMOS (SCMOS) logic style using modified field-effect transistors (FETs) to solve this problem, that is, to achieve high performance, utilizing the high carrier mobility in these materials, and to reduce the current leakage resulting from their small band gap. Conventional CMOS FETs are modified to have an asymmetric structure where an additional assistant gate is introduced near the drain to further lower the potential barrier in on-state and to increase the barrier in off-state. SCMOS ICs are constructed using these modified asymmetric CMOS FETs, which demonstrate perfect rail-to-rail output with negligible voltage loss and 3 orders of magnitude suppression of the static power consumption and an operating speed similar to or even higher than that of CMOS ICs. Here, SCMOS is demonstrated using carbon nanotubes, but, in principle, this logic style can be used in ICs based on any small-band-gap semiconductors to provide simultaneously high performance and low power consumption.

Ambipolar Deep-Subthreshold Printed-Carbon-Nanotube Transistors for Ultralow-Voltage and Ultralow-Power Electronics

The development of ultralow-power and easy-to-fabricate electronics with potential for large-scale circuit integration (i.e., complementary or complementary-like) is an outstanding challenge for emerging off-the-grid applications, e.g., remote sensing, "place-and-forget", and the Internet of Things. Herein we address this challenge through the development of ambipolar transistors relying on solution-processed polymer-sorted semiconducting carbon nanotube networks (sc-SWCNTNs) operating in the deep-subthreshold regime. Application of self-assembled monolayers at the active channel interface enables the fine-tuning of sc-SWCNTN transistors toward well-balanced ambipolar deep-subthreshold characteristics. The significance of these features is assessed by exploring the applicability of such transistors to complementary-like integrated circuits, with respect to which the impact of the subthreshold slope and flatband voltage on voltage and power requirements is studied experimentally and theoretically. As demonstrated with inverter and NAND gates, the ambipolar deep-subthreshold sc-SWCNTN approach enables digital circuits with complementary-like operation and characteristics including wide noise margins and ultralow operational voltages (≤0.5 V), while exhibiting record-low power consumption (≤1 pW/μm). Among thin-film transistor technologies with minimal material complexity, our approach achieves the lowest energy and power dissipation figures reported to date, which are compatible with and highly attractive for emerging off-the-grid applications.

Fabrication of Carbon Nanotube Thin Films for Flexible Transistors by Using a Cross-Linked Amine Polymer

Owing to their remarkable properties, single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) are expected to be used in various flexible electronics applications. To fabricate SWCNT channel layers for TFTs, solution-based film formation on a self-assembled monolayer (SAM) covered with amino groups is commonly used. However, this method uses highly oxidized surfaces, which is not suitable for flexible polymeric substrates. In this work, a solution-based SWCNT film fabrication using methoxycarbonyl polyallylamine (Moc-PAA) is reported. The NH₂ -terminated surface of the cross-linked Moc-PAA layer enables the formation of highly dense and uniform SWCNT networks on both rigid and flexible substrates. TFTs that use the fabricated SWCNT thin film exhibited excellent performance with small variations. The presented simple method to access SWCNT thin film accelerates the realization of flexible nanoelectronics.

An Electrically Actuated, Carbon-Nanotube-Based Biomimetic Ion Pump

Single-walled carbon nanotubes (SWCNTs) are well-established transporters of electronic current, electrolyte, and ions. In this work, we demonstrate an electrically actuated biomimetic ion pump by combining these electronic and nanofluidic transport capabilities within an individual SWCNT device. Ion pumping is driven by a solid-state electronic input, as Coulomb drag coupling transduces electrical energy from solid-state charge along the SWCNT shell to electrolyte inside the SWCNT core. Short-circuit ionic currents, measured without an electrolyte potential difference, exceed 1 nA and scale larger with increasing ion concentrations through 1 M, demonstrating applicability under physiological (∼140 mM) and saltwater (∼600 mM) conditions. The interlayer coupling allows ionic currents to be tuned with the source-drain potential difference and electronic currents to be tuned with the electrolyte potential difference. This combined electronic-nanofluidic SWCNT device presents intriguing applications as a biomimetic ion pump or component of an artificial membrane.

Molecular Junctions Inspired by Nature: Electrical Conduction through Noncovalent Nanobelts

Charge transport occurs in a range of biomolecular systems, whose structures have covalent and noncovalent bonds. Understanding from these systems have yet to translate into molecular junction devices. We design junctions which have hydrogen-bonds between the edges of a series of prototype noncovalent nanobelts (NCNs) and vary the number of donor-acceptors to study their electrical properties. From frontier molecular orbitals (FMOs) and projected density of state (DOS) calculations, we found these NCN dimer junctions to have low HOMO-LUMO gaps and states at the Fermi level, suggesting these are metallic-like systems. Their conductance properties were studied with nonequilibrium Green's functions density functional theory (NEGF-DFT) and was found to decrease with cooperative H-bonding, that is, the conductance decreased as the alternating donor-acceptors around the nanobelts attenuates to a uniform distribution in the H-bonding arrays. The latter gave the highest conductance of 51.3 × 10^-6 S and the Seebeck coefficient showed n-type (-36 to -39 μV K^-1) behavior, while the lower conductors with alternating H-bonds are p-type (49.7 to 204 μV K^-1). In addition, the NCNs have appreciable binding energies (19.8 to 46.1 kcal mol^-1), implying they could form self-assembled monolayer (SAM) heterojunctions leading to a polymeric network for long-range charge transport.

Synthesis and nonlinear optical properties of semiconducting single-walled carbon nanotubes at 1 μm

Herein, we synthesized and extracted pure semiconducting single-walled carbon nanotubes (s-SWCNTs). Moreover, the nonlinear optical (NLO) properties, such as saturable absorption, two-photon absorption coefficient, modulation depth and optical limitation, of s-SWCNTs were experimentally determined using a high-energy 1064 nm nanosecond (ns) laser. Compared with the common SWCNTs, the s-SWCNTs demonstrated lower saturation intensity and lower two-photon absorption (TPA) coefficient. The modulation depth of the s-SWCNTs was as high as 8.6%. Based on these parameters, the s-SWCNTs can be used as excellent saturable absorbers in pulsed laser applications.

Graphene-Si CMOS oscillators

Graphene field-effect transistors (GFETs) offer a possibility of exploiting unique physical properties of graphene in realizing novel electronic circuits. However, graphene circuits often lack the voltage swing and switchability of Si complementary metal-oxide-semiconductor (CMOS) circuits, which are the main building block of modern electronics. Here we introduce graphene in Si CMOS circuits to exploit favorable electronic properties of both technologies and realize a new class of simple oscillators using only a GFET, Si CMOS D latch, and timing RC circuit. The operation of the two types of realized oscillators is based on the ambipolarity of graphene, i.e., the symmetry of the transfer curve of GFETs around the Dirac point. The ambipolarity of graphene also allowed to turn the oscillators into pulse-width modulators (with a duty cycle ratio ∼1 : 4) and voltage-controlled oscillators (with a frequency ratio ∼1 : 8) without any circuit modifications. The oscillation frequency was in the range from 4 kHz to 4 MHz and limited only by the external circuit connections, rather than components themselves. The demonstrated graphene-Si CMOS hybrid circuits pave the way to the more widespread adoption of graphene in electronics.

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

A long-standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky-barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first-principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky-barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.

Conversion of Multi-layered MoTe2 Transistor Between P-Type and N-Type and Their Use in Inverter

Both p-type and n-type MoTe₂ transistors are needed to fabricate complementary electronic and optoelectronic devices. In this study, we fabricate air-stable p-type multi-layered MoTe₂ transistors using Au as electrode and achieve the conversion of p-type transistor to n-type by annealing it in vacuum. Temperature-dependent in situ measurements assisted by the results given by first-principle simulations indicate that n-type conductance is an intrinsic property, which is attributed to tellurium vacancies in MoTe₂, while the device in air experiences a charge transfer which is caused by oxygen/water redox couple and is converted to air-stable p-type transistor. Based on p-type and n-type multi-layered MoTe₂ transistors, we demonstrate a complementary inverter with gain values as high as 9 at V_DD = 5 V.

A study of the density of states of ZnCoO:H from resistivity measurements

Understanding the electronic band structure and density of states (DOS) of a material and their relationship to the associated electronic transport properties is the starting point for optimizing the performance of a device and its technological applications. In a hydrogenated Zn_0.8Co_0.2O (ZnCoO:H) film with an inverted thin-film transistor structure, we found ambipolar behavior, which is shown in many field-effect devices based on graphene, graphene nanoribbons, and organic semiconductors. In this study, to obtain information on the DOS of ZnCoO:H to explain the ambipolar behavior in terms of the carrier density and type, resistivity and magnetoresistance measurements of a ZnCoO:H film were performed at 5 K. Our proposed DOS representation of ZnCoO:H explains qualitatively the experimental observations of carrier density modulation and ambipolar behavior. First-principles calculations of the DOS of ZnCoO:H were in good agreement with the proposed DOS representation. Through a comparison of first-principles calculations and experimental data, evidence for the existence of Co–H–Co in ZnCoO:H is suggested.

Ambipolar behavior in a hydrogenated Zn_0.8Co_0.2O (ZnCoO:H) film is investigated via resistivity and magnetoresistance measurements and first-principles calculations of the DOS. Evidence for the existence of Co–H–Co in ZnCoO:H is suggested.

Polymer-based doping control for performance enhancement of wet-processed short-channel CNTFETs

The electrical transport properties of short-channel transistors based on single-walled carbon nanotubes (CNT) are significantly affected by bundling along with solution processing. We report that especially high off currents of CNT transistors are not only related to the incorporation of metallic CNTs but also to the incorporation of CNT bundles. By applying device passivation with poly(4-vinylpyridine), the impact of CNT bundling on the device performance can be strongly reduced due to increased gate efficiency as well as reduced oxygen and water-induced p-type doping, boosting essential field-effect transistor performance parameters by several orders of magnitude. Moreover, this passivation approach allows the hysteresis and threshold voltage of CNT transistors to be tuned.

Single Wall Carbon Nanotubes Based Cryogenic Temperature Sensor Platforms

We present an investigation consisting of single walled carbon nanotubes (SWCNTs) based cryogenic temperature sensors, capable of measuring temperatures in the range of 2-77 K. Carbon nanotubes (CNTs) due to their extremely small size, superior thermal and electrical properties have suggested that it is possible to create devices that will meet necessary requirements for miniaturization and better performance, by comparison to temperature sensors currently available on the market. Starting from SWCNTs, as starting material, a resistive structure was designed. Employing dropcast method, the carbon nanotubes were deposited over pairs of gold electrodes and in between the structure electrodes from a solution. The procedure was followed by an alignment process between the electrodes using a dielectrophoretic method. Two sensor structures were tested in cryogenic field down to 2 K, and the resistance was measured using a standard four-point method. The measurement results suggest that, at temperatures below 20 K, the temperature coefficient of resistance average for sensor 1 is 1.473%/K and for sensor 2 is 0.365%/K. From the experimental data, it can be concluded that the dependence of electrical resistance versus temperature can be approximated by an exponential equation and, correspondingly, a set of coefficients are calculated. It is further concluded that the proposed approach described here offers several advantages, which can be employed in the fabrication of a microsensors for cryogenic applications.

Ultra-Low-Power Design and Hardware Security Using Emerging Technologies for Internet of Things

In this review article for Internet of Things (IoT) applications, important low-power design techniques for digital and mixed-signal analog–digital converter (ADC) circuits are presented. Emerging low voltage logic devices and non-volatile memories (NVMs) beyond CMOS are illustrated. In addition, energy-constrained hardware security issues are reviewed. Specifically, light-weight encryption-based correlational power analysis, successive approximation register (SAR) ADC security using tunnel field effect transistors (FETs), logic obfuscation using silicon nanowire FETs, and all-spin logic devices are highlighted. Furthermore, a novel ultra-low power design using bio-inspired neuromorphic computing and spiking neural network security are discussed.

Carbon nanotube radio-frequency electronics

Carbon nanotube (CNT) is considered a promising material for radio-frequency (RF) applications, owing to its high carrier mobility and saturated drift velocity, as well as ultra-small intrinsic gate capacitance. Here, we review progress on CNT-based devices and integrated circuits for RF applications, including theoretical projection of RF performance of CNT-based devices, preparation of CNT materials, fabrication, optimization of RF field-effect transistors (FETs) structures, and ambipolar FET-based RF applications, and we outline challenges and prospects of CNT-based RF applications.

Control of tunnel barriers in multi-wall carbon nanotubes using focused ion beam irradiation

We have formed tunnel barriers in individual multi-wall carbon nanotubes using the Ga focused ion beam irradiation. The barrier height was estimated by the temperature dependence of the current (Arrhenius plot) and the current-voltage curves (Fowler-Nordheim plot). It is shown that the barrier height has a strong correlation with the barrier resistance that is controlled by the dose. Possible origins for the variation in observed barrier characteristics are discussed. Finally, the single electron transistor with two barriers is demonstrated.

Carbon Nanotubes in TiO2 Nanofiber Photoelectrodes for High-Performance Perovskite Solar Cells

1D semiconducting oxides are unique structures that have been widely used for photovoltaic (PV) devices due to their capability to provide a direct pathway for charge transport. In addition, carbon nanotubes (CNTs) have played multifunctional roles in a range of PV cells because of their fascinating properties. Herein, the influence of CNTs on the PV performance of 1D titanium dioxide nanofiber (TiO2 NF) photoelectrode perovskite solar cells (PSCs) is systematically explored. Among the different types of CNTs, single-walled CNTs (SWCNTs) incorporated in the TiO2 NF photoelectrode PSCs show a significant enhancement (≈40%) in the power conversion efficiency (PCE) as compared to control cells. SWCNTs incorporated in TiO2 NFs provide a fast electron transfer within the photoelectrode, resulting in an increase in the short-circuit current (Jsc) value. On the basis of our theoretical calculations, the improved open-circuit voltage (Voc) of the cells can be attributed to a shift in energy level of the photoelectrodes after the introduction of SWCNTs. Furthermore, it is found that the incorporation of SWCNTs into TiO2 NFs reduces the hysteresis effect and improves the stability of the PSC devices. In this study, the best performing PSC device constructed with SWCNT structures achieves a PCE of 14.03%.

Orthogonal Ambipolar Semiconductor Nanostructures for Complementary Logic Gates

We report orthogonal ambipolar semiconductors that exhibit hole and electron transport in perpendicular directions based on aligned films of nanocrystalline "shish-kebabs" containing poly(3-hexylthiophene) (P3HT) and N,N'-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI) as p- and n-type components, respectively. Polarized optical microscopy, scanning electron microscopy, and X-ray diffraction measurements reveal a high degree of in-plane alignment. Relying on the orientation of interdigitated electrodes to enable efficient charge transport from either the respective p- or n-channel materials, we demonstrate semiconductor films with high anisotropy in the sign of charge carriers. Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching behavior and a high noise margin of 80% of 1/2 Vdd. Moreover, complementary "NAND" and "NOR" logic gates were fabricated and found to exhibit excellent voltage transfer characteristics and low static power consumption. The ability to optimize the performance of these devices, simply by adjusting the solution concentrations of P3HT and PDI, makes this a simple and versatile method for preparing ambipolar organic semiconductor devices and high-performance logic gates. Further, we demonstrate that this method can also be applied to mixtures of PDI with another conjugated polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]) (PBTTT), with better hole transport characteristics than P3HT, opening the door to orthogonal ambipolar semiconductors with higher performance.

The impact of Cr adhesion layer on CNFET electrical characteristics

The effect of a Cr adhesion layer on the transfer characteristics of Cr/Au-contacted carbon nanotube field-effect transistors (CNFETs) based on individual single-walled carbon nanotubes (SWNTs) is presented in this paper. We show that a very thin Cr layer (≈0.4 nm) already has an impact on the carrier transport in Schottky-barrier-modulated CNFETs. The ratio of the p- and n-branch current is reduced by eight times when the Cr adhesion layer thickness is increased from 0 to 8 nm. We suggest a change in Schottky barrier height at the contact as the determining mechanism for this result. Additionally, superior lifetime of devices is observed even for non-passivated CNFETs with preserved clean SWNT/Cr/Au-contacts using Cr layer thinner than 2 nm. Our experiments show that the role of the adhesion layer in metal/nanotube contacts should be explicitly considered when designing CNTFET-based circuits, developing CNFET fabrication processes, and analyzing the corresponding properties of the electrical contacts.

Inkjet Printed Circuits on Flexible and Rigid Substrates Based on Ambipolar Carbon Nanotubes with High Operational Stability

Inkjet printed ambipolar transistors and circuits with high operational stability are demonstrated on flexible and rigid substrates employing semiconducting single-walled carbon nanotubes (SWCNTs). All patterns, which include electrodes, semiconductors, and vias, are realized by inkjet printing without the use of rigid physical masks and photolithography. An Al2O3 layer deposited on devices by atomic layer deposition (ALD) transforms p-type SWCNT thin-film transistors (TFTs) into ambipolar SWCNT TFTs and encapsulates them effectively. The ambipolar SWCNT TFTs have balanced electron and hole mobilities, which facilitates their use in multicomponent circuits. For example, a variety of logic gates and ring oscillators are demonstrated based on the ambipolar TFTs. The three-stage ring oscillator operates continuously for longer than 80 h under ambient conditions with only slight deviations in oscillation frequency. The successful demonstration of ambipolar devices by inkjet printing will enable a new class of circuits that utilize n-channel, p-channel, and ambipolar circuit components.

Fabrication of air-stable n-type carbon nanotube thin-film transistors on flexible substrates using bilayer dielectrics

Single-walled carbon nanotube (SWNT) thin-film transistors hold great potential for flexible electronics. However, fabrication of air-stable n-type devices by methods compatible with standard photolithography on flexible substrates is challenging. Here, we demonstrated that by using a bilayer dielectric structure of MgO and atomic layer deposited (ALD) Al2O3 or HfO2, air-stable n-type devices can be obtained. The mechanism for conduction type conversion was elucidated and attributed to the hole depletion in SWNT, the decrease of the trap state density by MgO assimilating adsorbed water molecules in the vicinity of SWNT, and the energy band bending because of the positive fixed charges in the ALD layer. The key advantage of the method is the relatively low temperature (120 or 90 °C) required here for the ALD process because we need not employ this step to totally remove the absorbates on the SWNTs. This advantage facilitates the integration of both p-type and n-type transistors through a simple lift off process and compact CMOS inverters were demonstrated. We also demonstrated that the doping of SWNTs in the channel plays a more important role than the Schottky barriers at the metal contacts in carbon nanotube thin-film transistors, unlike the situation in individual SWNT-based transistors.

Analysing black phosphorus transistors using an analytic Schottky barrier MOSFET model

Owing to the difficulties associated with substitutional doping of low-dimensional nanomaterials, most field-effect transistors built from carbon nanotubes, two-dimensional crystals and other low-dimensional channels are Schottky barrier MOSFETs (metal-oxide-semiconductor field-effect transistors). The transmission through a Schottky barrier-MOSFET is dominated by the gate-dependent transmission through the Schottky barriers at the metal-to-channel interfaces. This makes the use of conventional transistor models highly inappropriate and has lead researchers in the past frequently to extract incorrect intrinsic properties, for example, mobility, for many novel nanomaterials. Here we propose a simple modelling approach to quantitatively describe the transfer characteristics of Schottky barrier-MOSFETs from ultra-thin body materials accurately in the device off-state. In particular, after validating the model through the analysis of a set of ultra-thin silicon field-effect transistor data, we have successfully applied our approach to extract Schottky barrier heights for electrons and holes in black phosphorus devices for a large range of body thicknesses.

Conventional models of transistors are not applicable to devices made from nanomaterials because their operation is dominated by gate-dependent transmission through a Schottkybarrier. Here, the authors develop an analytical model and compare it to data taken from ultrathin silicon field-effect transistors.

End-bonded contacts for carbon nanotube transistors with low, size-independent resistance

Moving beyond the limits of silicon transistors requires both a high-performance channel and high-quality electrical contacts. Carbon nanotubes provide high-performance channels below 10 nanometers, but as with silicon, the increase in contact resistance with decreasing size becomes a major performance roadblock. We report a single-walled carbon nanotube (SWNT) transistor technology with an end-bonded contact scheme that leads to size-independent contact resistance to overcome the scaling limits of conventional side-bonded or planar contact schemes. A high-performance SWNT transistor was fabricated with a sub–10-nanometer contact length, showing a device resistance below 36 kilohms and on-current above 15 microampere per tube. The p-type end-bonded contact, formed through the reaction of molybdenum with the SWNT to form carbide, also exhibited no Schottky barrier. This strategy promises high-performance SWNT transistors, enabling future ultimately scaled device technologies.

Electron-Hole Confinement Symmetry in Silicon Quantum Dots

We report electrical transport measurements on a gate-defined ambipolar quantum dot in intrinsic silicon. The ambipolarity allows its operation as either an electron or a hole quantum dot of which we change the dot occupancy by 20 charge carriers in each regime. Electron-hole confinement symmetry is evidenced by the extracted gate capacitances and charging energies. The results demonstrate that ambipolar quantum dots offer great potential for spin-based quantum information processing, since confined electrons and holes can be compared and manipulated in the same crystalline environment.

Intense deep-blue electroluminescence from ITO/Y₂O₃/Ag structure

ITO/Y₂O₃/Ag devices were fabricated using Y₂O₃ films as insulator. Four intense and sharp lines with half-peak width of 4 nm were observed for the 293.78 nm InI, 316.10 nm InI, 444.82 nm InII and 403.07 nm InIII transitions. Luminescence mechanism was illustrated by cross-section of the devices based on the analysis of surface morphology. Under the action of strong electric field, the loss of K-shell electrons led to the occurrence of characteristic radiation of indium ions. In addition, the device with turn-on voltage of 10V demonstrates typical I-V diode characteristics. Moreover, Y₂O₃/In₂O₃ multiple films as the insulation layer instead of single Y₂O₃ films was found to improve the device performance with excellent CIE (x, y) coordinates (0.16, 0.03).

Carbon Nanotube Driver Circuit for 6 × 6 Organic Light Emitting Diode Display

Single-walled carbon nanotube (SWNT) is expected to be a very promising material for flexible and transparent driver circuits for active matrix organic light emitting diode (AM OLED) displays due to its high field-effect mobility, excellent current carrying capacity, optical transparency and mechanical flexibility. Although there have been several publications about SWNT driver circuits, none of them have shown static and dynamic images with the AM OLED displays. Here we report on the first successful chemical vapor deposition (CVD)-grown SWNT network thin film transistor (TFT) driver circuits for static and dynamic AM OLED displays with 6 × 6 pixels. The high device mobility of ~45 cm(2)V(-1)s(-1) and the high channel current on/off ratio of ~10(5) of the SWNT-TFTs fully guarantee the control capability to the OLED pixels. Our results suggest that SWNT-TFTs are promising backplane building blocks for future OLED displays.

Simultaneous Improvement of Hole and Electron Injection in Organic Field-effect Transistors by Conjugated Polymer-wrapped Carbon Nanotube Interlayers

Efficient charge injection is critical for flexible organic electronic devices such as organic light-emitting diodes (OLEDs) and field-effect transistors (OFETs). Here, we investigated conjugated polymer-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) as solution-processable charge-injection layers in ambipolar organic field-effect transistors with poly(thienylenevinylene-co-phthalimide)s. The interlayers were prepared using poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) or poly(9,9-dioctylfluorene) (PFO) to wrap s-SWNTs. In the contact-limited ambipolar OFETs, the interlayer led to significantly lower contact resistance (R_c) and increased mobilities for both holes and electrons. The resulting PTVPhI-Eh OFETs with PFO-wrapped s-SWNT interlayers showed very well-balanced ambipolar transport properties with a hole mobility of 0.5 cm²V^-1S^-1 and an electron mobility of 0.5 cm²V^-1S^-1 in linear regime. In addition, the chirality of s-SWNTs and kind of wrapping of conjugated polymers are not critical to improving charge-injection properties. We found that the improvements caused by the interlayer were due to the better charge injection at the metal/organic semiconductor contact interface and the increase in the charge concentration through a detailed examination of charge transport with low-temperature measurements. Finally, we successfully demonstrated complementary ambipolar inverters incorporating the interlayers without excessive patterning.

Wafer-level hysteresis-free resonant carbon nanotube transistors

We report wafer-level fabrication of resonant-body carbon nanotube (CNT) field-effect transistors (FETs) in a dual-gate configuration. An integration density of >10(6) CNTFETs/cm(2), an assembly yield of >80%, and nanoprecision have been simultaneously obtained. Through combined chemical and thermal treatments, hysteresis-free (in vacuum) suspended-body CNTFETs have been demonstrated. Electrostatic actuation by lateral gate and FET-based readout of mechanical resonance have been achieved at room temperature. Both upward and downward in situ frequency tuning has been experimentally demonstrated in the dual-gate architecture. The minuscule mass, high resonance frequency, and in situ tunability of the resonant CNTFETs offer promising features for applications in radio frequency signal processing and ultrasensitive sensing.

Origins and characteristics of the threshold voltage variability of quasiballistic single-walled carbon nanotube field-effect transistors

Ultrascaled transistors based on single-walled carbon nanotubes are identified as one of the top candidates for future microprocessor chips as they provide significantly better device performance and scaling properties than conventional silicon technologies. From the perspective of the chip performance, the device variability is as important as the device performance for practical applications. This paper presents a systematic investigation on the origins and characteristics of the threshold voltage (VT) variability of scaled quasiballistic nanotube transistors. Analysis of experimental results from variable-temperature measurement as well as gate oxide thickness scaling studies shows that the random variation from fixed charges present on the oxide surface close to nanotubes dominates the VT variability of nanotube transistors. The VT variability of single-tube transistors has a figure of merit that is quantitatively comparable with that of current silicon devices; and it could be reduced with the adoption of improved device passivation schemes, which might be necessary for practical devices incorporating multiple nanotubes, whose area normalized VT variability becomes worse due to the synergic effects from the limited surface coverage of nanotubes and the nonlinearity of the device off-state leakage current, as predicted by the Monte Carlo simulation.

Enhanced Hole Extraction in Perovskite Solar Cells Through Carbon Nanotubes

Here, we report the use of polymer-wrapped carbon nanotubes as a means to enhance charge extraction through undoped spiro-OMeTAD. With this approach a good solar cell performance is achieved without the implementation of conventional doping methods. We demonstrate that a stratified two-layer architecture of sequentially deposited layers of carbon nanotubes and spiro-OMeTAD, outperforms a conventional blend of the hole-conductor and the carbon nanotubes. We also provide insights into the mechanism of the rapid hole extraction observed in the two-layer approach.

Electrical contacts to individual SWCNTs: A review

Owing to their superior electrical characteristics, nanometer dimensions and definable lengths, single-walled carbon nanotubes (SWCNTs) are considered as one of the most promising materials for various types of nanodevices. Additionally, they can be used as either passive or active elements. To be integrated into circuitry or devices, they are typically connected with metal leads to provide electrical contacts. The properties and quality of these electrical contacts are important for the function and performance of SWCNT-based devices. Since carbon nanotubes are quasi-one-dimensional structures, contacts to them are different from those for bulk semiconductors. Additionally, some techniques used in Si-based technology are not compatible with SWCNT-based device fabrication, such as the contact area cleaning technique. In this review, an overview of the investigations of metal-SWCNT contacts is presented, including the principle of charge carrier injection through the metal-SWCNT contacts and experimental achievements. The methods for characterizing the electrical contacts are discussed as well. The parameters which influence the contact properties are summarized, mainly focusing on the contact geometry, metal type and the cleanliness of the SWCNT surface affected by the fabrication processes. Moreover, the challenges for widespread application of CNFETs are additionally discussed.

Ambipolar MoTe2 transistors and their applications in logic circuits

We report ambipolar charge transport in α-molybdenum ditelluride (MoTe2 ) flakes, whereby the temperature dependence of the electrical characteristics was systematically analyzed. The ambipolarity of the charge transport originated from the formation of Schottky barriers at the metal/MoTe2 contacts. The Schottky barrier heights as well as the current on/off ratio could be modified by modulating the electrostatic fields of the back-gate voltage (Vbg) and drain-source voltage (Vds). Using these ambipolar MoTe2 transistors we fabricated complementary inverters and amplifiers, demonstrating their feasibility for future digital and analog circuit applications.

High performance semiconducting enriched carbon nanotube thin film transistors using metallic carbon nanotubes as electrodes

High-performance solution-processed short-channel carbon nanotube (CNT) thin film transistors (TFTs) are fabricated using densely aligned arrays of metallic CNTs (m-CNTs) for the source and drain electrodes, while aligned arrays of semiconducting enriched CNTs (s-CNTs) are used as the channel material. The electrical transport measurements at room temperature show that using the m-CNT as the contact for the s-CNT array devices with a 2 μm channel length performed superior to those where the control Pd was the contact. The m-CNT contact devices exhibited a maximum (average) on-conductance of 36.5 μS (19.2 μS), a transconductance of 2.6 μS (1.2 μS), a mobility of 51 cm(2) V(-1) s(-1) (25 cm(2) V(-1) s(-1)), and a current on-off ratio of 1.1 × 10(6) (2.5 × 10(5)). These values are almost an order of magnitude higher than that of control Pd contact devices with the same channel length and s-CNT linear density. The low temperature charge transport measurements suggest that these improved performances are due to the m-CNT contact s-CNT devices having a lower Schottky barrier compared to the Pd contact s-CNT devices. We attribute this lower Schottky barrier to the unique geometry of our devices. In addition to using semiconducting enriched CNTs, our results suggest that using the metallic CNT as an electrode can significantly enhance the performance of CNT TFTs.

Optoelectronic devices based on electrically tunable p-n diodes in a monolayer dichalcogenide

The p-n junction is the functional element of many electronic and optoelectronic devices, including diodes, bipolar transistors, photodetectors, light-emitting diodes and solar cells. In conventional p-n junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Ambipolar semiconductors, such as carbon nanotubes, nanowires and organic molecules, allow for p-n junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here, we report ambipolar monolayer WSe2 devices in which two local gates are used to define a p-n junction within the WSe2 sheet. With these electrically tunable p-n junctions, we demonstrate both p-n and n-p diodes with ideality factors better than 2. Under optical excitation, the diodes demonstrate a photodetection responsivity of 210 mA W(-1) and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising values for a nearly transparent monolayer material in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe2. These devices provide a building block for ultrathin, flexible and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.

Tuning the threshold voltage of carbon nanotube transistors by n-type molecular doping for robust and flexible complementary circuits

Tuning the threshold voltage of a transistor is crucial for realizing robust digital circuits. For silicon transistors, the threshold voltage can be accurately controlled by doping. However, it remains challenging to tune the threshold voltage of single-wall nanotube (SWNT) thin-film transistors. Here, we report a facile method to controllably n-dope SWNTs using 1H-benzoimidazole derivatives processed via either solution coating or vacuum deposition. The threshold voltages of our polythiophene-sorted SWNT thin-film transistors can be tuned accurately and continuously over a wide range. Photoelectron spectroscopy measurements confirmed that the SWNT Fermi level shifted to the conduction band edge with increasing doping concentration. Using this doping approach, we proceeded to fabricate SWNT complementary inverters by inkjet printing of the dopants. We observed an unprecedented noise margin of 28 V at V(DD) = 80 V (70% of 1/2V(DD)) and a gain of 85. Additionally, robust SWNT complementary metal-oxide-semiconductor inverter (noise margin 72% of 1/2VDD) and logic gates with rail-to-rail output voltage swing and subnanowatt power consumption were fabricated onto a highly flexible substrate.