Controlled n-Type Doping of Carbon Nanotube Transistors by an Organorhodium Dimer

Molecular Doping Modulation and Applications of Structure-Sorted Single-Walled Carbon Nanotubes: A Review

Single-walled carbon nanotubes (SWCNTs) that have a reproducible distribution of chiralities or single chirality are among the most competitive materials for realizing post-silicon electronics. Molecular doping, with its non-destructive and fine-tunable characteristics, is emerging as the primary doping approach for the structure-controlled SWCNTs, enabling their eventual use in various functional devices. This review provides an overview of important advances in the area of molecular doping of structure-controlled SWCNTs and their applications. The first part introduces the underlying physical process of molecular doping, followed by a comprehensive survey of the commonly used dopants for SWCNTs to date. Then, it highlights how the convergence of molecular doping and structure-sorting strategies leads to significantly improved functionality of SWCNT-based field-effect transistor arrays, transparent electrodes in optoelectronics, thermoelectrics, and many emerging devices. At last, several challenges and opportunities in this field are discussed, with the hope of shedding light on promoting the practical application of SWCNTs in future electronics.

Carbon nanotube transistors: Making electronics from molecules

Semiconducting carbon nanotubes are robust molecules with nanometer-scale diameters that can be used in field-effect transistors, from larger thin-film implementation to devices that work in conjunction with silicon electronics, and can potentially be used as a platform for high-performance digital electronics as well as radio-frequency and sensing applications. Recent progress in the materials, devices, and technologies related to carbon nanotube transistors is briefly reviewed. Emphasis is placed on the most broadly impactful advancements that have evolved from single-nanotube devices to implementations with aligned nanotubes and even nanotube thin films. There are obstacles that remain to be addressed, including material synthesis and processing control, device structure design and transport considerations, and further integration demonstrations with improved reproducibility and reliability; however, the integration of more than 10,000 devices in single functional chips has already been realized.

Synthesis, structures, and reactivity of isomers of [RuCp*(1,4-(Me2N)2C6H4)]2

[RuCp*(1,3,5-R₃C₆H₃)]₂ {Cp* = η⁵-pentamethylcyclopentadienyl, R = Me, Et} have previously been found to be moderately air stable, yet highly reducing, with estimated D⁺/0.5D₂ (where D₂ and D⁺ represent the dimer and the corresponding monomeric cation, respectively) redox potentials of ca. -2.0 V vs. FeCp₂^+/0. These properties have led to their use as n-dopants for organic semiconductors. Use of arenes substituted with π-electron donors is anticipated to lead to even more strongly reducing dimers. [RuCp*(1-(Me₂N)-3,5-Me₂C₆H₃)]⁺PF₆^- and [RuCp*(1,4-(Me₂N)₂C₆H₄)]⁺PF₆^- have been synthesized and electrochemically and crystallographically characterized; both exhibit D⁺/D potentials slightly more cathodic than [RuCp*(1,3,5-R₃C₆H₃)]⁺. Reduction of [RuCp*(1,4-(Me₂N)₂C₆H₄)]⁺PF₆^- using silica-supported sodium-potassium alloy leads to a mixture of isomers of [RuCp*(1,4-(Me₂N)₂C₆H₄)]₂, two of which have been crystallographically characterized. One of these isomers has a similar molecular structure to [RuCp*(1,3,5-Et₃C₆H₃)]₂; the central C-C bond is exo,exo, i.e., on the opposite face of both six-membered rings from the metals. A D⁺/0.5D₂ potential of -2.4 V is estimated for this exo,exo dimer, more reducing than that of [RuCp*(1,3,5-R₃C₆H₃)]₂ (-2.0 V). This isomer reacts much more rapidly with both air and electron acceptors than [RuCp*(1,3,5-R₃C₆H₃)]₂ due to a much more cathodic D₂˙⁺/D₂ potential. The other isomer to be crystallographically characterized, along with a third isomer, are both dimerized in an exo,endo fashion, representing the first examples of such dimers. Density functional theory calculations and reactivity studies indicate that the central bonds of these two isomers are weaker than those of the exo,exo isomer, or of [RuCp*(1,3,5-R₃C₆H₃)]₂, leading to estimated D⁺/0.5D₂ potentials of -2.5 and -2.6 V vs. FeCp₂^+/0. At the same time the D₂˙⁺/D₂ potentials for the exo,endo dimers are anodically shifted relative to those of [RuCp*(1,3,5-R₃C₆H₃)]₂, resulting in much greater air stability than for the exo,exo isomer.

Printed carbon nanotube thin-film transistors: progress on printable materials and the path to applications

Printing technologies have attracted significant attention owing to their potential use in the low-cost manufacturing of custom or large-area flexible electronics. Among the many printable electronic materials that have been explored, semiconducting carbon nanotubes (CNTs) have shown increasing promise based on their exceptional electrical and mechanical properties, relative stability in air, and compatibility with several printing techniques to form semiconducting thin films. These attractive attributes make printed CNT thin films promising for applications including, but not limited to, sensors and display backplanes - at the heart of which is electronics' most versatile device: the transistor. In this review, we present a summary of recent advancements in the field of printed carbon nanotube thin-film transistors (CNT-TFTs). In addition to an introduction of different printing techniques, together with their strengths and limitations, we discuss key aspects of ink/material selection and processing of various device components, including the CNT channels, contacts, and gate insulators. It is clear that printed CNT-TFTs are rapidly advancing, but there remain challenges, which are discussed along with current techniques to resolve them and future developments towards practical applications from these devices. There has been interest in low-cost, printable transistors for many years and the CNT-TFTs show great promise for delivering, but will not become a reality without further research advancement.

Chirality-Enriched Carbon Nanotubes for Next-Generation Computing

For the past half century, silicon has served as the primary material platform for integrated circuit technology. However, the recent proliferation of nontraditional electronics, such as wearables, embedded systems, and low-power portable devices, has led to increasingly complex mechanical and electrical performance requirements. Among emerging electronic materials, single-walled carbon nanotubes (SWCNTs) are promising candidates for next-generation computing as a result of their superlative electrical, optical, and mechanical properties. Moreover, their chirality-dependent properties enable a wide range of emerging electronic applications including sub-10 nm complementary field-effect transistors, optoelectronic integrated circuits, and enantiomer-recognition sensors. Here, recent progress in SWCNT-based computing devices is reviewed, with an emphasis on the relationship between chirality enrichment and electronic functionality. In particular, after highlighting chirality-dependent SWCNT properties and chirality enrichment methods, the range of computing applications that have been demonstrated using chirality-enriched SWCNTs are summarized. By identifying remaining challenges and opportunities, this work provides a roadmap for next-generation SWCNT-based computing.

Modern microprocessor built from complementary carbon nanotube transistors

Electronics is approaching a major paradigm shift because silicon transistor scaling no longer yields historical energy-efficiency benefits, spurring research towards beyond-silicon nanotechnologies. In particular, carbon nanotube field-effect transistor (CNFET)-based digital circuits promise substantial energy-efficiency benefits, but the inability to perfectly control intrinsic nanoscale defects and variability in carbon nanotubes has precluded the realization of very-large-scale integrated systems. Here we overcome these challenges to demonstrate a beyond-silicon microprocessor built entirely from CNFETs. This 16-bit microprocessor is based on the RISC-V instruction set, runs standard 32-bit instructions on 16-bit data and addresses, comprises more than 14,000 complementary metal-oxide-semiconductor CNFETs and is designed and fabricated using industry-standard design flows and processes. We propose a manufacturing methodology for carbon nanotubes, a set of combined processing and design techniques for overcoming nanoscale imperfections at macroscopic scales across full wafer substrates. This work experimentally validates a promising path towards practical beyond-silicon electronic systems.

Fabrication of N-doped multidimensional carbon nanofibers for high-performance cortisol biosensors

Cortisol is an hormone that regulates blood pressure, glucose levels and carbohydrate metabolism in humans. Abnormal secretion of cortisol can cause various symptoms closely linked to psychological and physical health. In this study, high-performance field-effect transistor (FET)-based biosensors for cortisol detection were fabricated from N-doped multidimensional carbon nanofibers. Nanofiber morphology was controlled by tailoring the pressure conditions during vapor deposition polymerization (VDP). Thereafter, conductive channels of FET were completed by thermal annealing, acid treatment, and antibody attachment. Changes associated with chemical processes were characterized by various instruments. The resulting transducers exhibited a rapid response toward cortisol molecules with accurate selectivity, stable reusability, and high sensitivity. Minimum detection level were as low as 100 aM with a wide linear detection range of 100 aM to 10 nM due to the large surface area of the transducer and a correspondingly high number of antibody labels. The response and applicability of these cortisol biosensors were also assessed using saliva as a test matrix.

Tunable n-Type Doping of Carbon Nanotubes through Engineered Atomic Layer Deposition HfOX Films

Although digital systems fabricated from carbon-nanotube-based field-effect transistors (CNFETs) promise significant energy efficiency benefits, realizing these benefits requires a complementary CNFET technology, i.e., CNFET CMOS, comprising both PMOS and NMOS CNFETs. Furthermore, this CNFET CMOS process must be robust ( e.g., air-stable), tunable ( e.g., ability to control CNFET threshold voltages), and silicon CMOS compatible (to integrate within existing manufacturing facilities and process flows). Despite many efforts, such a silicon CMOS compatible CNT doping strategy for forming NMOS CNFETs does not exist. Techniques today are either not air-stable (using reactive low work function metals), not solid-state or silicon CMOS compatible (employing soluble molecular dopants in ionic solutions), or have not demonstrated precise control over the amount of doping (for setting threshold voltage,  V_T). Here, we demonstrate an electrostatic doping technique that meets all of these requirements. The key to our technique is leveraging atomic layer deposition (ALD) to encapsulate CNTs with nonstoichiometric oxides. We show that ALD allows for precise control of oxide stoichiometry, which translates to direct control of the amount of CNT doping. We experimentally demonstrate the ability to modulate the strength of the p-type conduction branch by >2500× (measured as the change in current at fixed bias), realize NMOS CNFETs with n-type conduction ∼500× stronger than p-type conduction (also measured by the relative current at fixed biases), and tune V_T over a ∼1.5 V range. Moreover, our technique is compatible with other doping schemes; as an illustration, we combine electrostatic doping and low work function contact engineering to achieve CNFET CMOS with symmetric NMOS and PMOS ( i.e., CNFET ON-current for NMOS and PMOS is within 6% of each other). Thus, this work realizes a solid-state, air-table, very large scale integration and silicon CMOS compatible doping strategy, enabling integration of CNFET CMOS within standard fabrication processes today.

Efficient n-Doping and Hole Blocking in Single-Walled Carbon Nanotube Transistors with 1,2,4,5-Tetrakis(tetramethylguanidino)ben-zene

Efficient, stable, and solution-based n-doping of semiconducting single-walled carbon nanotubes (SWCNTs) is highly desired for complementary circuits but remains a significant challenge. Here, we present 1,2,4,5-tetrakis(tetramethylguanidino)benzene (ttmgb) as a strong two-electron donor that enables the fabrication of purely n-type SWCNT field-effect transistors (FETs). We apply ttmgb to networks of monochiral, semiconducting (6,5) SWCNTs that show intrinsic ambipolar behavior in bottom-contact/top-gate FETs and obtain unipolar n-type transport with 3-5-fold enhancement of electron mobilities (approximately 10 cm² V^-1 s^-1), while completely suppressing hole currents, even at high drain voltages. These n-type FETs show excellent on/off current ratios of up to 10⁸, steep subthreshold swings (80-100 mV/dec), and almost no hysteresis. Their excellent device characteristics stem from the reduction of the work function of the gold electrodes via contact doping, blocking of hole injection by ttmgb²⁺ on the electrode surface, and removal of residual water from the SWCNT network by ttmgb protonation. The ttmgb-treated SWCNT FETs also display excellent environmental stability under bias stress in ambient conditions. Complementary inverters based on n- and p-doped SWCNT FETs exhibit rail-to-rail operation with high gain and low power dissipation. The simple and stable ttmgb molecule thus serves as an example for the larger class of guanidino-functionalized aromatic compounds as promising electron donors for high-performance thin film electronics.

Band-gap engineering of halogenated silicon nanowires through molecular doping

In this work, we address the effects of molecular doping on the electronic properties of fluorinated and chlorinated silicon nanowires (SiNWs), in comparison with those corresponding to hydrogen-passivated SiNWs. Adsorption of n-type dopant molecules on hydrogenated and halogenated SiNWs and their chemisorption energies, formation energies, and electronic band gap are studied by using density functional theory calculations. The results show that there are considerable charge transfers and strong covalent interactions between the dopant molecules and the SiNWs. Moreover, the results show that the energy band gap of SiNWs changes due to chemical surface doping and it can be further tuned by surface passivation. We conclude that a molecular based ex-situ doping, where molecules are adsorbed on the surface of the SiNW, can be an alternative path to conventional doping. Graphical abstract Molecular doping of halogenated silicon nanowires.

High-Performance Complementary Transistors and Medium-Scale Integrated Circuits Based on Carbon Nanotube Thin Films

Solution-derived carbon nanotube (CNT) network films with high semiconducting purity are suitable materials for the wafer-scale fabrication of field-effect transistors (FETs) and integrated circuits (ICs). However, it is challenging to realize high-performance complementary metal-oxide semiconductor (CMOS) FETs with high yield and stability on such CNT network films, and this difficulty hinders the development of CNT-film-based ICs. In this work, we developed a doping-free process for the fabrication of CMOS FETs based on solution-processed CNT network films, in which the polarity of the FETs was controlled using Sc or Pd as the source/drain contacts to selectively inject carriers into the channels. The fabricated top-gated CMOS FETs showed high symmetry between the characteristics of n- and p-type devices and exhibited high-performance uniformity and excellent scalability down to a gate length of 1 μm. Many common types of CMOS ICs, including typical logic gates, sequential circuits, and arithmetic units, were constructed based on CNT films, and the fabricated ICs exhibited rail-to-rail outputs because of the high noise margin of CMOS circuits. In particular, 4-bit full adders consisting of 132 CMOS FETs were realized with 100% yield, thereby demonstrating that this CMOS technology shows the potential to advance the development of medium-scale CNT-network-film-based ICs.

Inkjet printed circuits based on ambipolar and p-type carbon nanotube thin-film transistors

Ambipolar and p-type single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) are reliably integrated into various complementary-like circuits on the same substrate by inkjet printing. We describe the fabrication and characteristics of inverters, ring oscillators, and NAND gates based on complementary-like circuits fabricated with such TFTs as building blocks. We also show that complementary-like circuits have potential use as chemical sensors in ambient conditions since changes to the TFT characteristics of the p-channel TFTs in the circuit alter the overall operating characteristics of the circuit. The use of circuits rather than individual devices as sensors integrates sensing and signal processing functions, thereby simplifying overall system design.