2D Material Based Inks For Room Temperature Printed Electronics

Functional printing is a versatile mass production method that has gained considerable scientific and industrial attention in the past few years. A wide range of electronics from passive circuit components (e.g., resistors and interconnects) to high-performance transistors and high-quality displays can now be printed energy-, resource-, and cost-efficiently. In spite of the recent progresses in the formulation of inks and significant reductions in their production costs, most of the commercially available inks are still very expensive (e.g., silver inks), and limited to a few conductive and insulator materials that usually require high-temperature thermal treatments to become fully functional. To enable new applications, and further reduce the production costs, it is necessary to develop novel inks based on semiconductive and new conductive materials (with improved chemical or optical properties) that can become functional after drying (dry-only inks). Development of such inks is very challenging, and the few successful results have been mostly obtained by polymer-based inks, which suffer from cost and chemical-stability issues.Since 2004 and after the first successful isolation of single-layer pristine graphene, 2D materials have disrupted almost all fields of science and technology, from medicine, to electronics and beyond. Their unique morphology, diverse and widely-tunable physical and chemical properties, ease of synthesis, low-cost of production, and abundance of raw materials have brought up new possibilities, which were not conceivable before. Formulation of additive-free inks for room-temperature printing of electronics is one of such possibilities. In spite of their great potential, formulation of additive-free 2D-material-based inks has so far been limited to low-concentration inks that are not suitable for high-throughput manufacturing, which is one of the main incentives for printing of electronics. Furthermore, current 2D materials' synthesis and processing methods have either low yield, or are not scalable, which further limit their real-world applications. Here in this work, several processing and ink formulation strategies are reported, which can address the aforementioned problems and pave the way towards out of lab application of 2D materials.In chapter 1, after a brief introduction on different device fabrication methods, basics of liquid-phase processing, especially the topics that are related to ink formulation and printing, are concisely reviewed. In chapter 2, a universal strategy for additive-free formulation of pristine 2D materials-based inks, and their scalable production are reported. Being a universal approach, this method allows us to print advanced electronic devices consisting different types of semiconductive and conductive materials. MXenes are a recently discovered group of 2D materials with exceptional optical and electronic properties which can revolutionize the printed electronics industry. Due to their importance and being at their early stages of discovery, 2 chapters of this thesis (chapters 3 & 4) have been dedicated to their ink formulation challenges. In chapter 3, the possibility of using byproducts of MXene synthesis as a sustainable ink formulation strategy has been investigated, which can simultaneously address the financial and environmental issues of printing electronics. In chapter 4, the conventional MXene ink formulation strategies are revisited to enable their scalable printing.

All-printed low-power metal oxide gas sensors on polymeric substrates

Danick Briand, Saleem Khan

This paper presents a novel approach for the fabrication of miniaturized and fully printed metal-oxide (MOX) gas sensors on a polyimide (PI) substrate by using digital manufacturing of the functional layers, namely aerosol jet and inkjet printing technologies. We are reporting a stacking approach for all-printed MOX sensors for the first time. High resolution (similar to 20 mu m) printing of metallic patterns is enabled by aerosol jet of gold nanoparticles in solution, which leads to printed resistive thermal transducers with a reduced size of 500 x 500 mu m(2). The wide area printing feature of the aerosol jet is also applied for printing a thin (similar to 2 mu m) inter-dielectric PI layer in between the heater and gold electrodes. The gas sensing layer, i.e. Pd-doped tin oxide (SnO2) nanoparticles, is deposited through drop on demand inkjet printing. Proper operation of the printed sensors is evaluated in both dry and humid conditions under reducing and oxidizing gases i.e. CO and NO2, respectively. All the tests are performed at 250 degrees C, produced by the integrated micro-hotplate at a low power consumption of similar to 78 mW. The chemo-resistive responses of the sensors towards both the gases are found to be in the acceptable range as compared to conventional metal-oxide gas sensor responses. These results are very promising for future integration of MOX gas sensors notably in smart printed electronics, disposable systems and wearables.

IOP PUBLISHING LTD2019

Impact of Strain and Alloy Disorder on the Electronic Properties of III-Nitride Based Two-dimensional Electron Gases

Pirouz Sohi

Over the past 20 years, III-nitrides (GaN, AlN, InN and their alloys) have proven to be an excellent material group for electronic devices, in particular, for high electron mobility transistors (HEMTs) operating at high frequency and high power. This is mainly thanks to the wide band gap of III-nitrides and the spontaneous polarization along the technologically relevant (0001) direction. Heteroepitaxy of III-nitride heterostructures enables the formation of two-dimensional electron gases (2DEGs) with high carrier densities (> 10^13 cm^-2) and high electron mobilities (2000 cm^2V^-1s^-1). The heterostructures usually consist of AlGaN, InAlN or AlN barriers grown epitaxially on GaN buffers. Devices based on these structures exhibit excellent performance and have reached technological maturity, enabling notably the commercialization of AlGaN/GaN HEMTs. Despite the success of III-nitride HEMTs thus far, the full potential of the material group for electronics has not yet been unleashed. The aim of this thesis is to investigate two novel heterostructure designs and to determine the physical mechanisms limiting their electronic properties. This was done by epitaxial growth followed by characterization of material and electronic properties. The first studied heterostructure was AlN/GaN/AlN where the GaN channel is fully strained to AlN. In order to achieve pseudomorphic growth of GaN on AlN, the onset of strain relaxation, i.e., the critical thickness, is of paramount importance. Furthermore, the impact of growth parameters such as temperature and initial dislocation density need to be considered. Over the past years several, research groups have reported on the electronic properties of ultra-thin GaN channels on AlN. Interestingly, all reported 2DEGs suffer from a low electron mobility with values usually below 600 cm^2V^-1s^-1. In this thesis, the strain relaxation and critical thickness of GaN on AlN were determined. Furthermore, the origin of low electron mobility for AlN/GaN/AlN heterostructures and in general thin-GaN channels grown on AlN were systematically studied. The second heterostructure that was investigated during this thesis was based on InGaN channels. In this case, the 2DEG is formed in an In-rich InGaN layer. The low electron effective mass of InN (0.05 m0) compared to GaN (0.2 m0) should theoretically give rise to a higher electron mobility. However, for In-rich channels alloy disorder scattering is particularity strong in III-nitrides. This has been shown both by optical measurements and theoretical calculations based on the localization landscape theory. Surprisingly, high electron mobilities have been reported in literature for high In content InGaN channels. In these cases, the electron mobility appears not to be limited by alloy disorder scattering. During this thesis, the electron mobility of InGaN channels was determined as a function of In content. Furthermore, the unintentional growth of GaN interlayers is proposed as the possible origin for the reported high electron mobilities in literature.

EPFL2019

Functional Ink Formulation for Printing and Coating of Graphene and Other 2D Materials: Challenges and Solutions

Sina Abdolhosseinzadeh, Jakob Heier, Mohammad Jafarpour, Frank Nüesch

The properties of 2D materials are unparalleled when compared to their 3D counterparts; many of these properties are a consequence of their size reduction to only a couple of atomic layers. Metallic, semiconducting, and insulating types can be found and form a platform for a new generation of devices. Among the possible methods to utilize 2D materials, functional printing has emerged as a strong contender because inks can be directly formulated from dispersions obtained by liquid-phase exfoliation. Printed graphene-based devices are shifting from laboratory applications toward real-world and mass-producible systems going hand in hand with a good understanding of suitable exfoliation methods for the targeted type of ink. Such a clear picture does not yet exist for hexagonal boron nitride (h-BN), the transition metal dichalcogenides (TMDs), and black phosphorous (BP). Rather, reports of applications of these 2D materials in printed devices are scattered throughout the literature, not yet adding to a comprehensive and full understanding of the relevant parameters. This perspective starts with a summary of the most important features of inks from exfoliated graphene. For h-BN, the TMDs, and BP, the characteristic properties when exfoliated from solution and strategies to formulate inks are summarized.

WILEY2022

Impact of Strain and Alloy Disorder on the Electronic Properties of III-Nitride Based Two-dimensional Electron Gases

Pirouz Sohi

EPFL2019