Division of Interdisciplinary Research

2D Piezotronics: Performance to Functionality

2D Piezotronics: Performance to Functionality Yarajena, Sai Saraswathi In the pursuit of interactive electronic devices, there is a need for smart materials which can serve multiple functionalities. 2D (two-dimensional) layered materials have gained attention in semiconductor technology because of their versatile electrical and optical properties. Furthermore, some materials exhibit piezoelectricity at 2D scale and can withstand enormous strain. These properties make them suitable as smart materials involving electromechanical signals. In the literature, materials which are semiconducting and piezoelectric are termed piezotronic (piezo+electronic) materials. Theoretical studies have indicated many materials as piezoelectric in 2D form. However, experimental tools to investigate the extent of piezoelectric coupling in 2D materials are limited, and their relevance for piezotronics has not been studied in detail. This dissertation presents some key aspects of 2D Piezotronics for improved performance and to achieve additional functionalities with heterojunctions. The work constitutes proposing a technique to estimate piezoelectric coupling coefficients, choice of flexible substrates for piezotronics, methods to reduce the charge screening effects, measurement strategies to extract the actual piezoelectric output from the bending measurements, and the study of heterojunctions for rectifying behaviour. In this work, Molybdenum disulfide (MoS2) is used as active piezoelectric material. In the initial part of the work, I propose a technique to estimate in-plane piezoelectric coupling quantitatively for 2D materials. The method involves a novel approach for in-plane field excitation in lateral Piezo force microscopy (PFM). Contact resonance gain of the tip-sample system is leveraged to measure the piezoelectric coupling coefficients in a few pm/V to sub pm/V range. However, I have shown that operating PFM at contact resonance can cause pseudo piezoelectric signals. Therefore, a detailed methodology for signal calibration and electrostatic background subtraction is developed in this work. The technique is verified by estimating the in-plane piezoelectric coupling coefficients (d11) for freely suspended MoS2 of one to five atomic layers. The technique presented is useful in estimating the piezoelectric coupling strengths in emerging 2D materials. Piezotronic devices are made on flexible substrates for practical applications. Fabrication on flexible substrates often poses great difficulties in handling them, depositing inorganic materials, and carrying out lithography processes. I propose the commercially available nano flex film as a prospective substrate for piezotronics. Carrying out fabrication on these substrates is as seamless as that on rigid substrates. Substrates such as PET, Nano flex and TPU can be used for low-temperature (<150 deg C) applications. Kapton is one of the flexible substrates that can handle higher temperatures(>200 deg C). However, they tend to twist when heated, making the fabrication difficult. I have proposed a gel-based bonding for the Kapton substrates wherein the debonding process is automatic. The method is helpful for the fabrication of 2D material devices on Kapton. Besides selecting the substrates, suitable base layers and passivation techniques are studied to reduce the charge screening effects and thus improve the performance of piezotronic devices. It is verified that open circuit voltages and strain gauge factors obtained for the current monolayer MoS2 device on SiO2 are three folds higher than those presented in the literature. A simple measurement setup which does not require probe needles or wire bonding is developed for the bending strain measurements. The open circuit voltage and short circuit current signals obtained from a single 2D material device are very small. The noise signals that originate from various triboelectric and electrostatic sources of the measurement setup can be of similar magnitude. Consequently, the electrical outputs from these devices during bending measurements are often misinterpreted. Thus, it is essential to analyse various noise sources in bending measurements. I then discuss ways to reduce the background noise and identify the valid piezoelectric output. Finally, I have studied some homogeneous and heterogeneous junctions of MoS2 to achieve good rectifying junction behaviour, which can add extra functionalities for piezotronics. The rectification ratio values as high as 5000 could be achieved at 1 V bias. Besides the rectifying ratio, I have observed that the heterojunctions of MoS2 and MoTe2 have superior piezoelectric behaviour compared to other 2D material junctions reported so far with open circuit voltages as high as ~1 V and peak power density of ~200 mW/m2 at 0.44% bending strain. Formation of the p-n and Schottky junction hybrid in MoS2-MoTe2 heterojunction could achieve high rectification ratios and open circuit voltages and is fascinating for further study.

3D Packaging for Integration of Heterogeneous Systems

3D Packaging for Integration of Heterogeneous Systems Nittala, Pavani Vamsi Krishna With several new applications getting developed around wearable technologies for Internet of Things (IoT), there has been a growing need for development of the miniaturized systems. Emerging applications in healthcare, structural monitoring, consumer accessories, etc are fuelling the need for these miniaturized hybrid systems. Such micro-nano systems will be enabled through the development of heterogeneous integration technologies that will allow co-packaging of several chips with different functionalities in a single vertical 3D stack. Therefore, the consumer electronics industry has initiated development of 3D integration of CMOS devices in vertical stacks which are electrically interconnected using thru-silicon-via (TSV) technology. This technology is however not suitable for stacks having a complex combination of GaN-HEMT’s, MEMS, microfluidics, optical devices and CMOS. Moreover, due to the cross-contamination issues, most of these devices are never accepted in the standard silicon CMOS foundries. To address these issues, we have developed innovative processing technologies that would allow 3D packaging by the post fab vertical stacking technique, suitable for the packaging industry. In the First Part of the thesis, we have developed processing technologies for the 3D stacking of the homogenous silicon systems. Using them, we have demonstrated a low temperature process to transfer MOS devices on ultra-thin silicon layers (1.5 μm) from a parent substrate to a foreign substrate or stack. In order to enable this transfer, we have analysed and resolved the associated stress issues. Furthermore, we demonstrate three-layer stacking of the ultra-thin silicon layers with functional MOSFET’s in each layer. We extensively characterize the changes in the device performance, which arise due to the transfer process. In the Second Part of the work, we have demonstrated an approach for stacking the III-nitride-on-Si HEMTs and Si-MOSFETs on to a copper substrate. The developed process flow offers a significant improvement in the device behaviour due to the transfer to a thermally conducting substrate like copper. The functional AlGaN/GaN epi-layer stack from the HEMT-on-silicon wafer is lifted-off and bonded to a copper substrate using novel Cu-In bond. Next, an ultra-thin silicon layer (~1.5 μm) with functional NMOS transistors fabricated in-house, on an SOI wafer are separated from the parent SOI wafer and then stacked over the GaN devices already bonded on the copper substrate, using cost-effective epoxy bonding approach. The devices are characterised to study the improvements in their performance. In the Third Part, we have demonstrated a 3D integration method for miniaturisation of hybrid systems. Using this 3D packaging technique, a fluorescence-sensing platform consisting of (i) a silicon photodetector, (ii) plastic optical filters, (iii) commercial LED and (iv) a glass micro-heater chip is demonstrated. We have resolved several fabrication challenges related to planarization, stacking and interconnection of these divergent chips. The above process flow developed in this work, can be scaled to stack a larger number of layers for achieving more complicated systems with enhanced functionality and applications. Finally, we have demonstrated interconnection methodologies using the nonconventional inkjet printing technique for via filling to enable identical die size stacking.

4D Printing of Polymer Composites to Engineer Resorbable and Deployable Implants

4D Printing of Polymer Composites to Engineer Resorbable and Deployable Implants Choudhury, Saswat Medical implants include products that are used to replace or restore the function of any damaged organ or tissue inside the body. The current generation of implants are mostly made from non-degradable materials and are implanted via open surgeries. The patients often need to undergo multiple surgical interventions in case of implant failure or secondary complications. This inevitably leads to increased pain, hospitalization times, and higher costs. Resorbable medical implants offer potential advantages, as they completely degrade inside the body into non-toxic byproducts in a timely fashion, thereby avoiding multiple revision procedures. A few examples exist of resorbable implants used in clinics, such as polymeric bone fixation devices. On the other hand, deployable implants provide the feature of implantation via minimally invasive procedures. Typical examples of such implants include Nitinol-based stents, which are non-degradable and require surgical procedures for initial implantation. However, combining both features, including absorbability and deployability, in a single implant is scarce. A typical way of achieving this is to use an absorbable smart material that can be triggered using benign stimulation methods to deploy at the site minimally invasively. The thesis encompasses engineering such absorbable and stimuli-responsive polymer composites fabricated into customized designs of potential implants using design- aided additive manufacturing. The first part of the thesis aims at design optimization, using an absorbable shape memory polymer (SMP) that can exhibit shape recovery at physiological temperatures. Using anisotropic design principles, the SMP was programmed to generate out- of-plane shape deformations in as-printed planar structures, such as hollow tubes with cells in the lumen. The later parts of the thesis deal with engineering the shape memory polymer with functional nanoparticles that are responsive to non-contact stimuli, such as alternating magnetic field, or near-infrared (NIR) for remotely deployable medical implants. Lastly, as a proof-of- concept, a biliary stent is fabricated using absorbable polymer and later endowed with NIR- responsiveness and imaging potential for potential image-guided deployment. Chapter 1 presents a comprehensive literature survey on resorbable and deployable medical implants, focusing on the materials and manufacturing employed. The need for resorbable polymers, additive manufacturing (3D printing and 4D printing) as potential manufacturing techniques, the role of design and smart materials on shape change, and different stimulation methods to trigger shape change are outlined. A brief overview of the potential biomedical applications is also presented, emphasizing biliary stents and deployable bone tissue scaffolds prepared by such materials. In Chapter 2, details of experimental methods and characterization techniques employed to carry out the work in this thesis are mentioned. Chapter 3 presents a dual shape-morphing strategy comprising design-aided additive manufacturing and shape memory properties of polymers to fabricate complex shapes, such as cellularized hollow tubes. FEA simulations are used to predict the different types of shape deformations, such as out-of-plane bending and twisting in the as-printed structures by varying infill angles during printing. Hollow tubes of varying lengths and diameters can be obtained by quantitative predictions from the simulations. The obtained tubes can then be cellularized by utilizing the shape memory property of the polymer, resulting in cellularized vascular grafts for improved patency. Chapter 4 utilizes an alternating magnetic field as a stimulation for fabricating deployable tissue scaffolds. The shape memory polymer was reinforced with magnetic nanoparticles to realize remote heating under an alternating magnetic field and at physiological temperatures. The as-printed magnetic scaffolds could be deployed minimally invasively following shape recovery from deformed shapes into original shapes under magnetic heating. Furthermore, the composites exhibited in vitro osteogenesis, cytocompatibility, and in vivo compatibility. Thus, the 4D printed shape memory magnetic nanocomposite presented here could be an excellent candidate biomaterial for engineering deployable scaffolds and medical implants, among other implantable applications. In Chapter 5, near-infrared (NIR) light was used as a potential stimulation to realize in situ deployable tissue scaffolds. The shape memory polymer was nanoengineered with photothermal nanofillers to fabricate composites by 3D printing. The composites demonstrated significantly higher osteogenic potential in vitro, as revealed by the significantly enhanced alkaline phosphatase (ALP) secretion and mineral deposition compared to the neat polymer. Intraoperative deployability and in vivo bone regeneration ability of the composites were demonstrated using self-fitting scaffolds in critical-sized cranial bone defects in rabbits. The composite scaffolds fabricated here offer an innovative strategy for minimally invasive deployment to fit irregular and complex tissue defects for bone tissue regeneration. In Chapter 6, bioresorbable biliary stents were fabricated as a different class of implants using 3D printing and elcectrospinning to obtain covered stents of custom lattice designs. The stents were tested for in vitro mechanical properties and degradation properties and their deployment was assessed in pre-clinical porcine model using the Seldinger technique. In Chapter 7, NIR-responsive composites were prepared using photothermal nanoparticles with imaging potential using fluorescent imaging and photoacoustic imaging techniques. In Chapter 8, the overall findings of the work are summarized, and a brief perspective on the scope of future work based on the findings of this thesis is also presented. Taken together, this thesis presents a combination of engineering materials, design optimization, smart use stimulation strategies, for the rationale development of bioresorbable and deployable implants.

A protocol for the Bi-directional connectivity of peers in the presence of heterogeneous internet address space

A protocol for the Bi-directional connectivity of peers in the presence of heterogeneous internet address space Sreedhar, Y The number of hosts connecting to the Internet is growing day by day, and the available statistics show that this growth is exponential. This growth has made IP address a scarce resource. IPv6 is proposed as a long term solution to the problem of depletion of IP address space. IPv6 has many advantages over IPv4, and is carefully designed to serve the future requirements. But the migration from IPv4 to IPv6 can’t be done over night, because it requires extensive modifications to the existing infrastructure. It will take around 15 to 20 years to deploy IPv6 fully. Some temporary solutions to solve the problem have been proposed in the literature. One such temporary solution is NAT (Network Address Translator). Network Address Translator (NAT) gateway provides a way of utilizing the available IP addresses effectively. NAPT or Network Address Port Translator is one popular flavor of NAT gateway which uses one IP address and multiplexes many hosts to connect to the Internet. On the Internet every connection is identified by four parameters. They are source IP address, source port number, destination IP address and destination port number. In each and every out-going packet the NAPT changes source IP address to its own IP address, but the source port is changed to a unique value that is used to associate incoming packets with the private network address. NAPT maintains full outbound connectivity and denies the internal hosts the ability to receive unsolicited inbound connections. That is, connection initiation can be done only from the internal host. The host outside the NAT can’t initiate a connection to the host behind the NAT. This denial of inbound connections is to provide security to the internal hosts. The NAPT is fully transparent to the end-users, since the source ports in the IP packet headers are translated on the fly. In the normal client server architecture, the lack of inbound connectivity is fine because all servers are located in the public domain. But the current world is moving towards the peer to peer networking. In peer to peer networking, the distinction between a client and a server vanishes and, every host acts as both client and server. This is to exploit the databases distributed among the peers. Under these circumstances every host demands inbound connectivity. The major application of the peer to peer networking is in file sharing. This thesis proposes a novel technique to solve the problem of lack of inbound connectivity of NAT, by modifying the peer to peer file sharing protocol, while retaining the advantage of NAT in utilizing the IP address space effectively. In the proposed method all inbound IP packets are sent to the NAT gateway, and prior to that, NAT gateway is instructed, as to which private IP host these packets should go inside its own domain. For this to happen the initiator should know both the IP addresses of the NAT and the responder which is behind the NAT. This is done by modifying the directory service at the central host, which contains the details about the location of files. The modifications needed at the NAT gateway and to the peer to peer networking protocol to solve the lack of inbound connectivity problem of NAT, are presented in this thesis. Organization of Thesis The thesis is organized in 5 chapters as follows: Chapter I gives an elaborate account of present status of the Internet. The problem of scarcity of IPv4 address space is discussed. The temporary and permanent solutions to solve this problem is discussed. The problem of lack of inbound connectivity of NAT, which gets more attention in peer to peer networking is stated. Here the scope of the work is defined. Chapter II deals with a survey of the existing solutions to alleviate the lack of inbound connectivity problem of the NAT. The positive and negative aspects associated with each of these solutions are discussed. Chapter III explains the proposed solution to solve the lack of inbound connectivity on the NAT. Chapter IV deals with the implementation details of the proposed solution. Since the proposed solution is a protocol, all the packet formations used and all fields are explained in this chapter. Chapter V summarizes the contributions made by the thesis. Suggestions for future work is also included in this chapter.