Jonathan Roberts

Photonic Crystals for Enhanced Light Extraction from 2D Materials

In recent years, a range of two-dimensional transition metal dichalcogenides (TMDs) have been studied, and remarkable optical and electronic characteristics have been demonstrated. Furthermore, the weak interlayer van der Waals interaction allows TMDs to adapt to a range of substrates. Unfortunately, the photons emitted from these TMD monolayers are difficult to efficiently collect into simple optics, reducing the practicality of these materials. The realization of on-chip optical devices for quantum information applications requires structures that maximize optical extraction efficiency while also minimizing substrate loss. In this work we propose a photonic crystal cavity based on silicon rods that allows maximal spatial and spectral coupling between TMD monolayers and the cavity mode. Finite difference time domain simulations revealed that TMDs coupled to this type of cavity have highly directional emission toward the collection optics, as well as up to 400% enhancement in luminescence intensity, compa...

Optical identification using imperfections in 2D materials

The ability to uniquely identify an object or device is important for authentication. Imperfections, locked into structures during fabrication, can be used to provide a fingerprint that is challenging to reproduce. In this paper, we propose a simple optical technique to read unique information from nanometer-scale defects in 2D materials. Flaws created during crystal growth or fabrication lead to spatial variations in the bandgap of 2D materials that can be characterized through photoluminescence measurements. We show a simple setup involving an angle-adjustable transmission filter, simple optics and a CCD camera can capture spatially-dependent photoluminescence to produce complex maps of unique information from 2D monolayers. Atomic force microscopy is used to verify the origin of the optical signature measured, demonstrating that it results from nanometer-scale imperfections. This solution to optical identification with 2D materials could be employed as a robust security measure to prevent counterfeiting.

Extracting random numbers from quantum tunnelling through a single diode

Random number generation is crucial in many aspects of everyday life, as online security and privacy depend ultimately on the quality of random numbers. Many current implementations are based on pseudo-random number generators, but information security requires true random numbers for sensitive applications like key generation in banking, defence or even social media. True random number generators are systems whose outputs cannot be determined, even if their internal structure and response history are known. Sources of quantum noise are thus ideal for this application due to their intrinsic uncertainty. In this work, we propose using resonant tunnelling diodes as practical true random number generators based on a quantum mechanical effect. The output of the proposed devices can be directly used as a random stream of bits or can be further distilled using randomness extraction algorithms, depending on the application.

Large-Area Heterostructures from Graphene and Encapsulated Colloidal Quantum Dots via the Langmuir–Blodgett Method

This work explores the assembly of large-area heterostructures comprised of a film of silica-encapsulated, semiconducting colloidal quantum dots, deposited via the Langmuir-Blodgett method, sandwiched between two graphene sheets. The luminescent, electrically insulating film served as a dielectric, with the top graphene sheet patterned into an electrode and successfully used as a top gate for an underlying graphene field-effect transistor. This heterostructure paves the way for developing novel hybrid optoelectronic devices through the integration of 2D and 0D materials.

Langmuir-Blodgett Deposition of 2D Materials for Unique Identification

There are many instances in the field of security where optically identifiable tags are used, examples of these include holograms, special inks/prints and conventional anti-tamper taggants. Unfortunately, there are a range of problems with these existing systems, none more so than their ease of clonability and lack of individuality. Thus, it is of great societal and technological significance to develop an optically addressable analogue of an UNO/PUF-like device that can overcome these problems by relying on quantum confinement and thus, the local atomic environment of the system. In this chapter, the possibility of this is suggested by using two-dimensional materials known as transition metal dichalcogenides (TMDs), which contain a direct band-gap in the visible range and therefore emit light that could be detected efficiently by a standard silicon CCD.

Building Optoelectronic Heterostructures with Langmuir-Blodgett Deposition

The flexibility of the Langmuir-Blodgett technique for the deposition of unconventional materials has already been highlighted in the previous chapter. This ability, coupled with the ease of depositing such materials onto a range of substrates and its long-range order, results in the outstanding ability to design and fabricate novel heterostructures for optoelectronic device applications. In this chapter, the use of this method for the fabrication of complex heterostructures comprising of semiconducting colloidal quantum dots (CQDs) sandwiched between graphene sheets, is shown for the first time.

An Introduction to Security Based on Physical Disorder

The ever-growing number of connected smart devices, programs and data brings with it a growing demand to ensure the security and reliability of these systems. This problem is now a significant challenge for all of society, as these devices have become completely pervasive in everyday life. Example uses include carrying out financial transactions, communicating with other people, monitoring people’s health and interacting with the environment. As these devices fulfil critical tasks, one of the core requirements that needs to be addressed lies in their secure authentication, identification and integrity checking. This chapter introduces a strategy that has emerged over recent years, which utilises inherent, hard-to-clone randomness of physically disordered systems to define the secure identity of a system.

An Introduction to Semiconductors and Quantum Confinement

In this thesis, atomically imperfect structures will be studied to investigate if such systems can provide adequate variation for unique identification, as the sensitive effects of quantum confinement exhibited in these low-dimensional structures is directly related to the order on this scale. Utilising the measurement of a quantum effect amplifies the influence of these atomic-scale defects, and provides a simple way to measure them simply and reliably. This work will lay the foundations for UNOs and PUFs comprising of such structures to be developed.

Sample Preparation and Experimental Techniques

In this chapter, the techniques that have been employed for the following experimental chapters will be discussed. The methodologies used to fabricate the resonant tunneling diodes (RTDs) studied in Chap. 4 are discussed before describing the details of the electronic probe station measurements that were used to characterise them. The Langmuir-Blodgett technique used to fabricate 2D and OD thin films is then described in detail. Finally, the characterisation techniques used throughout the work are described.

Unique Identification with Resonant Tunneling Diodes

Electronic devices have become extremely prominent in everyday life, and it is now commonplace for them to control critical tasks, such as managing financial transactions. Therefore, it is of key importance that these devices can be securely identified to prevent illegitimate parties mimicking themselves as genuine, and gaining access to sensitive information. The popular methods of identification currently in use rely on the end user providing some information about themselves, such as a fingerprint or a password, but these authentication methods are known to be extremely vulnerable. Identities can also be provided by systems that exploit physical disorder, but there is a growing need for the security to be as robust as physically possible. Resonant tunneling diodes (RTDs), can provide such an uncomplicated measurement of identity, corresponding to the straightforward measurement of the macroscopic current that passes through the device.