Physics of Nanosystems

Ludwig-Maximilians-Universität München

Faculty of Physics

Geschwister-Scholl-Platz 1

80539 Munich

Group Webpage


Research focus: quantum transport, quantum nano-systems

Our work is dedicated to understanding the fundamental electronic properties of nanoscopic materials. We especially focus on novel systems such as organic semiconductors and 2-dimensional materials such as graphene. Examples of topics we are interested in are:

Quantum transport in bilayer graphene

Bilayer graphene is a fascinating system, since an applied electric field perpendicular to the layers can control the “which-layer” index. In the quantum-Hall regime, this allows to identify the underlying order of the individual filling factors. Bilayer graphene is not only fascinating at high magnetic fields. For example, we have identified that the ground state of ultraclean bilayer graphene is gapped due to the exchange interaction. Theory predicts several interesting possible ordered ground states that have experimentally not been conclusively identified.

Electronic properties of organic semiconductors and transition metal dichalcogenides

We are interested in the intrinsic electronic properties of high-performance organic small molecule and polymer semiconductors. Mostly, we rely on temperature dependent charge transport measurements to identify the electronic properties of such materials. For example, temperature and bias dependent charge transport has allowed us to entangle the contributions of contact and flake resistance of a MoS2 multilayer to the overall transistor performance.


Quantum anomalous Hall octet driven by orbital magnetism in bilayer graphene

F.R. Geisenhof, F. Winterer, A.M. Seiler, J. Lenz, T. Xu, F. Zhang, R.T. Weitz

Nature 598, 53–58 (2021).

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The quantum anomalous Hall (QAH) effect-a macroscopic manifestation of chiral band topology at zero magnetic field-has been experimentally realized only by the magnetic doping of topological insulators(1-3) and the delicate design of moire heterostructures(4-8). However, the seemingly simple bilayer graphene without magnetic doping or moire engineering has long been predicted to host competing ordered states with QAH effects(9-11). Here we explore states in bilayer graphene with a conductance of 2 e(2) h(-1) (where e is the electronic charge and h is Planck's constant) that not only survive down to anomalously small magnetic fields and up to temperatures of five kelvin but also exhibit magnetic hysteresis. Together, the experimental signatures provide compelling evidence for orbital-magnetism-driven QAH behaviour that is tunable via electric and magnetic fields as well as carrier sign. The observed octet of QAH phases is distinct from previous observations owing to its peculiar ferrimagnetic and ferrielectric order that is characterized by quantized anomalous charge, spin, valley and spin-valley Hall behaviour(9).

DOI: 10.1038/s41586-021-03849-w

Synthesis of large-area rhombohedral few-layer graphene by chemical vapor deposition on copper

C. Bouhafs, S. Pezzini, F.R. Geisenhof , N. Mishra, V. Mišeikis, Y. Niu, C. Struzzi, R.T. Weitz, A.A. Zakharov, S. Forti, C. Coletti

Carbon 177, 282-290 (2021).

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Rhombohedral-stacked few-layer graphene (FLG) displays peculiar electronic properties that could lead to phenomena such as high-temperature superconductivity and magnetic ordering. To date, experimental studies have been mainly limited by the difficulty in isolating rhombohedral FLG with thickness exceeding 3 layers and device-compatible size. In this work, we demonstrate the synthesis and transfer of rhombohedral graphene with thickness up to 9 layers and areas up to ∼50 μm2. The domains of rhombohedral FLG are identified by Raman spectroscopy and are found to alternate with Bernal regions within the same crystal in a stripe-like configuration. Near-field nano-imaging further confirms the structural integrity of the respective stacking orders. Combined spectroscopic and microscopic analyses indicate that rhombohedral-stacking formation is strongly correlated to the underlying copper step-bunching and emerges as a consequence of interlayer displacement along preferential crystallographic orientations. The growth and transfer of rhombohedral FLG with the reported thickness and size shall facilitate the observation of predicted unconventional physics and ultimately add to its technological relevance.

DOI: 10.1016/j.carbon.2021.02.082

Charge Traps in All-Inorganic CsPbBr3 Perovskite Nanowire Field-Effect Phototransistors

F. Winterer, L.S. Walter, J. Lenz, S. Seebauer, Y. Tong, L. Polavarapu, R.T. Weitz

Advanced electronic Materials 7 (6), 2100105 (2021).

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All-inorganic halide perovskite materials have recently emerged as outstanding materials for optoelectronic applications. However, although critical for developing novel technologies, the influence of charge traps on charge transport in all-inorganic systems still remains elusive. Here, the charge transport properties in cesium lead bromide, nanowire films are probed using a field-effect transistor geometry. Field-effect mobilities of μFET = 4 × 10−3 cm−2 V−1 s−1 and photoresponsivities in the range of R = 25 A W−1 are demonstrated. Furthermore, charge transport both with and without illumination is investigated down to cryogenic temperatures. Without illumination, deep traps dominate transport and the mobility freezes out at low temperatures. Despite the presence of deep traps, when illuminating the sample, the field-effect mobility increases by several orders of magnitude and even phonon-limited transport characteristics are visible. This can be seen as an extension to the notion of “defect tolerance” of perovskite materials that has solely been associated with shallow traps. These findings provide further insight in understanding charge transport in perovskite materials and underlines that managing deep traps can open up a route to optimizing optoelectronic devices such as solar cells or phototransistors operable also at low light intensities.

DOI: 10.1002/aelm.202100105

High-Performance Vertical Organic Transistors of Sub-5 nm Channel Length

J. Lenz, A.M. Seiler, F.R. Geisenhof, F. Winterer, K. Watanabe, T. Taniguchi, R.T. Weitz

Nano Letters 21 (10), 4430–4436 (2021).

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Miniaturization of electronic circuits increases their overall performance. So far, electronics based on organic semiconductors has not played an important role in the miniaturization race. Here, we show the fabrication of liquid electrolyte gated vertical organic field effect transistors with channel lengths down to 2.4 nm. These ultrashort channel lengths are enabled by using insulating hexagonal boron nitride with atomically precise thickness and flatness as a spacer separating the vertically aligned source and drain electrodes. The transistors reveal promising electrical characteristics with output current densities of up to 2.95 MA cm–2 at −0.4 V bias, on–off ratios of up to 106, a steep subthreshold swing of down to 65 mV dec–1 and a transconductance of up to 714 S m–1. Realizing channel lengths in the sub-5 nm regime and operation voltages down to 100 μV proves the potential of organic semiconductors for future highly integrated or low power electronics.

DOI: 10.1021/acs.nanolett.1c01144

Ionic liquid gating of single-walled carbon nanotube devices with ultra-short channel length down to 10nm

A. Jannisek, J. Lenz, F. del Giudice, M. Gaulke, F. Pyatkov, S. Dehm, F. Hennrich, L. Wei, Y. Chen, A. Fediai, M. Kappes, W. Wenzel, R. Krupke, R.T. Weitz

Applied Physics Letters 118 (6), 063101 (2021).

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Ionic liquids enable efficient gating of materials with nanoscale morphology due to the formation of a nanoscale double layer that can also follow strongly vaulted surfaces. On carbon nanotubes, this can lead to the formation of a cylindrical gate layer, allowing an ideal control of the drain current even at small gate voltages. In this work, we apply ionic liquid gating to chirality-sorted (9, 8) carbon nanotubes bridging metallic electrodes with gap sizes of 20nm and 10nm. The single-tube devices exhibit diameter-normalized current densities of up to 2.57mA/mu m, on-off ratios up to 10(4), and a subthreshold swing down to 100mV/dec. Measurements after long vacuum storage indicate that the hysteresis of ionic liquid gated devices depends not only on the gate voltage sweep rate and the polarization dynamics but also on charge traps in the vicinity of the carbon nanotube, which, in turn, might act as trap states for the ionic liquid ions. The ambipolar transfer characteristics are compared with calculations based on the Landauer-Buttiker formalism. Qualitative agreement is demonstrated, and the possible reasons for quantitative deviations and possible improvements to the model are discussed. Besides being of fundamental interest, the results have potential relevance for biosensing applications employing high-density device arrays.


Locally-triggered hydrophobic collapse induces global interface self-cleaning in van-der-Waals heterostructures at room-temperature

S. Wakolbinger, F.R. Geisenhof, F. Winterer, S. Palmer, J.G. Crimmann, K. Watanabe, T. Taniguchi, F. Trixler, R.T. Weitz

2D Materials 7 (3), 035002 (2020).

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Mutual relative orientation and well defined, uncontaminated interfaces are the key to obtain van-der-Waals heterostacks with defined properties. Even though the van-der-Waals forces are known to promote the 'self-cleaning' of interfaces, residue from the stamping process, which is often found to be trapped between the heterostructure constituents, can interrupt the interlayer interaction and therefore the coupling. Established interfacial cleaning methods usually involve high-temperature steps, which are in turn known to lead to uncontrolled rotations of layers within fragile heterostructures. Here, we present an alternative method feasible at room temperature. Using the tip of an atomic force microscope (AFM), we locally control the activation of interlayer attractive forces, resulting in the global removal of contaminants from the interface (i.e. the contaminants are also removed in regions several mu m away from the line touched by the AFM tip). By testing combinations of various hydrophobic van-der-Waals materials, mild temperature treatments, and by observing the temporal evolution of the contaminant removal process, we identify that the AFM tip triggers a dewetting-induced hydrophobic collapse and the van-der-Waals interaction is driving the cleaning process. We anticipate that this process is at the heart of the known 'self-cleaning' mechanism. Our technique can be utilized to controllably establish interlayer close coupling between a stack of van-der-Waals layers, and additionally allows to pattern and manipulate heterostructures locally for example to confine material into nanoscopic pockets between two van-der-Waals materials.

DOI: 10.1088/2053-1583/ab7bfc

Flexible low-voltage high-frequency organic thin-film transistors

J.W. Borchert, U. Zschieschang, F. Letzkus, M. Giorgio, R.T. Weitz, M. Caironi, J.N. Burghartz, S. Ludwigs, H. Klauk

Science Advances 6 (21), eaaz5156 (2020).

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The primary driver for the development of organic thin-film transistors (TFTs) over the past few decades has been the prospect of electronics applications on unconventional substrates requiring low-temperature processing. A key requirement for many such applications is high-frequency switching or amplification at the low operating voltages provided by lithium-ion batteries (similar to 3 V). To date, however, most organic-TFT technologies show limited dynamic performance unless high operating voltages are applied to mitigate high contact resistances and large parasitic capacitances. Here, we present flexible low-voltage organic TFTs with record static and dynamic performance, including contact resistance as small as 10, on/off current ratios as large as 10(10), subthreshold swing as small as 59 mV/decade, signal delays below 80 ns in inverters and ring oscillators, and transit frequencies as high as 21 MHz, all while using an inverted coplanar TFT structure that can be readily adapted to industry-standard lithographic techniques.

DOI: 10.1126/sciadv.aaz5156

Anisotropic Strain-Induced Soliton Movement Changes Stacking Order and Band Structure of Graphene Multilayers: Implications for Charge Transport

F.R: Geisenhof, F. Winterer, S. Wakolbinger, T.D. Gokus, Y.C. Durmaz, D. Priesack, J. Lenz, F. Keilmann, K. Watanabe, T. Taniguchi, R. Guerrero-Aviles, M. Pelc, A. Ayuela, R.T. Weitz

ACS Applied Nano Materials 2 (9), 6067-6075 (2019).

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The crystal structure of solid-state matter greatly affects its electronic properties. For example, in multilayer graphene, precise knowledge of the lateral layer arrangement is crucial, since the most stable configurations, Bernal and rhombohedral stacking, exhibit very different electronic properties. Nevertheless, both stacking orders can coexist within one flake, separated by a strain soliton that can host topologically protected states. Clearly, accessing the transport properties of the two stackings and the soliton is of high interest. However, the stacking orders can transform into one another, and therefore, the seemingly trivial question of how reliable electrical contact can be made to either stacking order can a priori not be answered easily. Here, we show that manufacturing metal contacts to multilayer graphene can move solitons by several ism, unidirectionally enlarging Bernal domains due to arising mechanical strain. Furthermore, we also find that during dry transfer of multilayer graphene onto hexagonal boron nitride, such a transformation can happen. Using density functional theory modeling, we corroborate that anisotropic deformations of the multilayer graphene lattice decrease the rhombohedral stacking stability. Finally, we have devised systematics to avoid soliton movement, and how to reliably realize contacts to both stacking configurations, which will aid to reliably access charge transport in both stacking configurations.

DOI: 10.1021/acsanm.9b01603

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