Lene Gammelgaard

The hot pick-up technique for batch assembly of van der Waals heterostructures

The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.

Graphene transport properties upon exposure to PMMA processing and heat treatments

The evolution of grapheneʼs electrical transport properties due to processing with the polymer polymethyl methacrylate (PMMA) and heat are examined in this study. The use of stencil (shadow mask) lithography enables fabrication of graphene devices without the usage of polymers, chemicals or heat, allowing us to measure the evolution of the electrical transport properties during individual processing steps from the initial as-exfoliated to the PMMA-processed graphene. Heating generally promotes the conformation of graphene to SiO2 and is found to play a major role for the electrical properties of graphene while PMMA residues are found to be surprisingly benign. In accordance with this picture, graphene devices with initially high carrier mobility tend to suffer a decrease in carrier mobility, while in contrast an improvement is observed for low carrier mobility devices. We explain this by noting that flakes conforming poorly to the substrate will have a higher carrier mobility which will however be reduced as heat treatment enhance the conformation. We finally show the electrical properties of graphene to be reversible upon heat treatments in air up to 200 °C.

Quality assessment of graphene: Continuity, uniformity, and accuracy of mobility measurements

With the increasing availability of large-area graphene, the ability to rapidly and accurately assess the quality of the electrical properties has become critically important. For practical applications, spatial variability in carrier density and carrier mobility must be controlled and minimized. We present a simple framework for assessing the quality and homogeneity of large-area graphene devices. The field effect in both exfoliated graphene devices encapsulated in hexagonal boron nitride and chemical vapor-deposited (CVD) devices was measured in dual current–voltage configurations and used to derive a single, gate-dependent effective shape factor, β, for each device. β is a sensitive indicator of spatial homogeneity that can be obtained from samples of arbitrary shape. All 50 devices investigated in this study show a variation (up to tenfold) in β as a function of the gate bias. Finite element simulations suggest that spatial doping inhomogeneity, rather than mobility inhomogeneity, is the primary cause of the gate dependence of β, and that measurable variations of β can be caused by doping variations as small as 1010 cm−2. Our results suggest that local variations in the position of the Dirac point alter the current flow and thus the effective sample shape as a function of the gate bias. We also found that such variations lead to systematic errors in carrier mobility calculations, which can be revealed by inspecting the corresponding β factor.

Graphene antidot lattice transport measurements

We investigate graphene devices patterned with a narrow band of holes perpendicular to the current flow, a few-row graphene antidot lattice (FR-GAL). Theoretical reports suggest that a FR-GAL can have a bandgap with a relatively small reduction of the transmission compared to what is typical for antidot arrays devices. Graphene devices were fabricated using 100 keV electron beam lithography (EBL) for nanopatterning as well as for defining electrical contacts. Patterns with hole diameter and neck widths of order 30 nm were produced, which is the highest reported pattern density of antidot lattices in graphene reported defined by EBL. Electrical measurements showed that devices with one and five rows exhibited field effect mobility of ∼100 cm2/Vs, while a larger number of rows, around 40, led to a significant reduction of field effect mobility (<5 cm2/Vs). The carrier mobility was measured as a function of temperature, with the low-temperature behaviour being well described by variable range hopping, indicating the transport to be dominated by disorder.

Micromechanical sensors for the measurement of biopolymer degradation

We present microcantilever-based sensors for the characterization of biopolymer degradation by enzymes. Thin films of Poly(L-lactide) (PLLA) were spray-coated onto SU-8 cantilevers with well-known material properties and dimensions. The micromechanical sensors were immersed in solutions of proteinase K to investigate enzymatic degradation of PLLA. A decrease of the resonance frequency after immersion indicated degradation of the biopolymer coating and allowed the estimation of the degradation rate at a specific enzyme concentration.

Conductance quantization suppression in the quantum Hall regime

Conductance quantization is the quintessential feature of electronic transport in non-interacting mesoscopic systems. This phenomenon is observed in quasi one-dimensional conductors at zero magnetic field B, and the formation of edge states at finite magnetic fields results in wider conductance plateaus within the quantum Hall regime. Electrostatic interactions can change this picture qualitatively. At finite B, screening mechanisms in narrow, gated ballistic conductors are predicted to give rise to an increase in conductance and a suppression of quantization due to the appearance of additional conduction channels. Despite being a universal effect, this regime has proven experimentally elusive because of difficulties in realizing one-dimensional systems with sufficiently hard-walled, disorder-free confinement. Here, we experimentally demonstrate the suppression of conductance quantization within the quantum Hall regime for graphene nanoconstrictions with low edge roughness. Our findings may have profound impact on fundamental studies of quantum transport in finite-size, two-dimensional crystals with low disorder.Conductance quantization is the hallmark of non-interacting confined systems. The authors show that the quantization in graphene nanoconstrictions with low edge disorder is suppressed in the quantum Hall regime. This is explained by the addition of new conductance channels due to electrostatic screening.

Flexible SiO2 cantilevers for torsional self-aligning micro scale four-point probes

In order to successfully measure the conductivity of a sample with a four-point probe, good alignment of the electrodes to the sample is important to establish even contact pressure and contact areas of the electrodes. By incorporating a hinge in a microfabricated SiO2 mono-cantilever the ability to compensate for misalignment is improved at a cost of reduced spring constant. Analytical calculations, numerical simulations on cantilever deflection and comparison with experimental results indicate that a reasonable compromise between torsional flexibility and overall spring constant can be achieved by proper dimensioning and placement of the hinge. Furthermore, it is shown that polymeric macro scale cantilever models can provide a fast and reliable understanding of the mechanical deflection properties of microfabricated SiO2 cantilevers.

Quantitative optical mapping of two-dimensional materials

The pace of two-dimensional materials (2DM) research has been greatly accelerated by the ability to identify exfoliated thicknesses down to a monolayer from their optical contrast. Since this process requires time-consuming and error-prone manual assignment to avoid false-positives from image features with similar contrast, efforts towards fast and reliable automated assignments schemes is essential. We show that by modelling the expected 2DM contrast in digitally captured images, we can automatically identify candidate regions of 2DM. More importantly, we show a computationally-light machine vision strategy for eliminating false-positives from this set of 2DM candidates through the combined use of binary thresholding, opening and closing filters, and shape-analysis from edge detection. Calculation of data pyramids for arbitrarily high-resolution optical coverage maps of two-dimensional materials produced in this way allows the real-time presentation and processing of this image data in a zoomable interface, enabling large datasets to be explored and analysed with ease. The result is that a standard optical microscope with CCD camera can be used as an analysis tool able to accurately determine the coverage, residue/contamination concentration, and layer number for a wide range of presented 2DMs.

High-quality graphene flakes exfoliated on a flat hydrophobic polymer

We show that graphene supported on a hydrophobic and flat polymer surface results in flakes with extremely low doping and strain as assessed by their Raman spectroscopic characteristics. We exemplify this technique by micromechanical exfoliation of graphene on flat poly(methylmethacrylate) layers and demonstrate Raman peak intensity ratios I(2D)/I(G) approaching 10, similar to pristine freestanding graphene. We verify that these features are not an artifact of optical interference effects occurring at the substrate: they are similarly observed when varying the substrate thickness and are maintained when the environment of the graphene flake is completely changed, by encapsulating preselected flakes between hexagonal boron nitride layers. The exfoliation of clean, pristine graphene layers directly on flat polymer substrates enables high performance, supported, and non-encapsulated graphene devices for flexible and transparent optoelectronic studies. We additionally show that the access to a clean and supported graphene source leads to high-quality van der Waals heterostructures and devices with reproducible carrier mobilities exceeding 50 000 cm2 V−1 s−1 at room temperature.

Cantilever Sensors for Measurement of Biopolymer Degradation

The idea behind the project is that biopolymers can be used in the future as lids on microscopic medicine boxes. The biopolymer is environmentally friendly, as it degrades naturally in the environments in the human body. If the biopolymer is used for medicine boxes, various medicines could be given in smaller doses, making it cheaper and reducing the risk of potentially secondary effects. In the project it will be investigated whether it is possible to measure the degradation of biopolymer, using cantilevers on chips with a microscopic scale.