Bernd Gotsmann

Nanoscale Three-Dimensional Patterning of Molecular Resists by Scanning Probes

Patterning a Molecular Glass Lithographic patterning for device fabrication is usually based on initiating polymerization reactions with photons or electrons in a molecular resist. However, patterning can be achieved by mechanically removing a hard resist with scanning probe microscopy tips, but in many cases the resolution is low and excess material is left on the surface. Pires et al. (p. 732, published online 22 April) found that thin films of organic molecules could form glasses through weak interactions and be patterned to tens of nanometers with a heated scanning probe tip. These patterns could be transferred to other substrates or sculpted into three-dimensional shapes by successive rounds of patterning. A molecular glass can be patterned to dimensions of tens of nanometers with a heated scanning probe tip. For patterning organic resists, optical and electron beam lithography are the most established methods; however, at resolutions below 30 nanometers, inherent problems result from unwanted exposure of the resist in nearby areas. We present a scanning probe lithography method based on the local desorption of a glassy organic resist by a heatable probe. We demonstrate patterning at a half pitch down to 15 nanometers without proximity corrections and with throughputs approaching those of Gaussian electron beam lithography at similar resolution. These patterns can be transferred to other substrates, and material can be removed in successive steps in order to fabricate complex three-dimensional structures.

Ultralow nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like carbon

Understanding friction and wear at the nanoscale is important for many applications that involve nanoscale components sliding on a surface, such as nanolithography, nanometrology and nanomanufacturing. Defects, cracks and other phenomena that influence material strength and wear at macroscopic scales are less important at the nanoscale, which is why nanowires can, for example, show higher strengths than bulk samples. The contact area between the materials must also be described differently at the nanoscale. Diamond-like carbon is routinely used as a surface coating in applications that require low friction and wear because it is resistant to wear at the macroscale, but there has been considerable debate about the wear mechanisms of diamond-like carbon at the nanoscale because it is difficult to fabricate diamond-like carbon structures with nanoscale fidelity. Here, we demonstrate the batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy. This material is known to possess low friction in humid conditions, and we find that, at the nanoscale, it is three orders of magnitude more wear-resistant than silicon under ambient conditions. A wear rate of one atom per micrometre of sliding on SiO(2) is demonstrated. We find that the classical wear law of Archard does not hold at the nanoscale; instead, atom-by-atom attrition dominates the wear mechanisms at these length scales. We estimate that the effective energy barrier for the removal of a single atom is approximately 1 eV, with an effective activation volume of approximately 1 x 10(-28) m.

Probe-based ultrahigh-density storage technology

Ultrahigh storage densities can be achieved by using a thermomechanical scanning-probe-based data-storage approach to write, read back, and erase data in very thin polymer films. High data rates are achieved by parallel operation of large two-dimensional arrays of cantilevers that can be batch fabricated by silicon-surface micromachining techniques. The very high precision required to navigate the storage medium relative to the array of probes is achieved by microelectromechanical system (MEMS)- based x and y actuators. The ultrahigh storage densities offered by probe-storage devices pose a significant challenge in terms of both control design for nanoscale positioning and read-channel design for reliable signal detection. Moreover, the high parallelism necessitates new dataflow architectures to ensure high performance and reliability of the system. In this paper, we present a small-scale prototype system of a storage device that we built based on scanning-probe technology. Experimental results of multiple sectors, recorded using multiple levers at 840 Gb/in2 and read back without errors, demonstrate the functionality of the prototype system. This is the first time a scanning-probe recording technology has reached this level of technical maturity, demonstrating the joint operation of all building blocks of a storage device.

Transport Properties of a Single-Molecule Diode

Charge transport through single diblock dipyrimidinyl diphenyl molecules consisting of a donor and acceptor moiety was measured in the low-bias regime and as a function of bias at different temperatures using the mechanically controllable break-junction technique. Conductance histograms acquired at 10 mV reveal two distinct peaks, separated by a factor of 1.5, representing the two orientations of the single molecule with respect to the applied bias. The current-voltage characteristics exhibit a temperature-independent rectification of up to a factor of 10 in the temperature range between 300 and 50 K with single-molecule currents of 45-70 nA at ±1.5 V. The current-voltage characteristics are discussed using a semiempirical model assuming a variable coupling of the molecular energy levels as well as a nonsymmetric voltage drop across the molecular junction, thus shifting the energy levels accordingly. The excellent agreement of the data with the proposed model suggests that the rectification originates from an asymmetric Coulomb blockade in combination with an electric-field-induced level shifting.

Tuning the Light Emission from GaAs Nanowires over 290 meV with Uniaxial Strain

Strain engineering has been used to increase the charge carrier mobility of complementary metal-oxide-semiconductor transistors as well as to boost and tune the performance of optoelectronic devices, enabling wavelength tuning, polarization selectivity and suppression of temperature drifts. Semiconducting nanowires benefit from enhanced mechanical properties, such as increased yield strength, that turn out to be beneficial to amplify strain effects. Here we use photoluminescence (PL) to study the effect of uniaxial stress on the electronic properties of GaAs/Al0.3Ga0.7As/GaAs core/shell nanowires. Both compressive and tensile mechanical stress were applied continuously and reversibly to the nanowire, resulting in a remarkable decrease of the bandgap of up to 296 meV at 3.5% of strain. Raman spectra were measured and analyzed to determine the axial strain in the nanowire and the Poisson ratio in the <111> direction. In both PL and Raman spectra, we observe fingerprints of symmetry breaking due to anisotropic deformation of the nanowire. The shifts observed in the PL and Raman spectra are well described by bulk deformation potentials for band structure and phonon energies. The fact that exceptionally high elastic strain can be applied to semiconducting nanowires makes them ideally suited for novel device applications that require a tuning of the band structure over a broad range.

Dynamic superlubricity and the elimination of wear on the nanoscale

One approach to ultrahigh-density data storage involves the use of arrays of atomic force microscope probes to read and write data on a thin polymer film, but damage to the ultrasharp silicon probe tips caused by mechanical wear has proved problematic. Here, we demonstrate the effective elimination of wear on a tip sliding on a polymer surface over a distance of 750 m by modulating the force acting on the tip-sample contact. Friction measurements as a function of modulation frequency and amplitude indicate that a reduction of friction is responsible for the reduction in wear to below our detection limit. In addition to its relevance to data storage, this approach could also reduce wear in micro- and nanoelectromechanical systems and other applications of scanning probe microscopes.

Quantitative thermometry of nanoscale hot spots.

A method is described to quantify thermal conductance and temperature distributions with nanoscale resolution using scanning thermal microscopy. In the first step, the thermal resistance of the tip-surface contact is measured for each point of a surface. In the second step, the local temperature is determined from the difference between the measured heat flux for heat sources switched on and off. The method is demonstrated using self-heating of silicon nanowires. While a homogeneous nanowire shows a bell-shaped temperature profile, a nanowire diode exhibits a hot spot centered near the junction between two doped segments.

Carbon nanotube tips for thermomechanical data storage

We report the demonstration of thermomechanical data storage in a poly(methylmethacrylate) film using a multiwalled carbon nanotube (MWCNT) tip. Indentation densities of >250 Gbits/in2 are achieved. The power efficiency of indent writing with MWCNT tips is found to be higher than that of conventional silicon tips owing to better heat transfer at the tip–polymer interface.

Inducing a direct-to-pseudodirect bandgap transition in wurtzite GaAs nanowires with uniaxial stress

Many efficient light-emitting devices and photodetectors are based on semiconductors with, respectively, a direct or indirect bandgap configuration. The less known pseudodirect bandgap configuration can be found in wurtzite (WZ) semiconductors: here electron and hole wave-functions overlap strongly but optical transitions between these states are impaired by symmetry. Switching between bandgap configurations would enable novel photonic applications but large anisotropic strain is normally needed to induce such band structure transitions. Here we show that the luminescence of WZ GaAs nanowires can be switched on and off, by inducing a reversible direct-to-pseudodirect band structure transition, under the influence of a small uniaxial stress. For the first time, we clarify the band structure of WZ GaAs, providing a conclusive picture of the energy and symmetry of the electronic states. We envisage a new generation of devices that can simultaneously serve as efficient light emitters and photodetectors by leveraging the strain degree of freedom.

High resolution vacuum scanning thermal microscopy of HfO2 and SiO2

We present scanning thermal microscopy (SThM) measurements on a sample consisting of regions of 3nm thick HfO2 film and 2nm thick SiO2 on a silicon substrate. The experiments were preformed in high vacuum conditions using microfabricated silicon cantilevers with sharp heatable tips, facilitating the unprecedented achievement of a lateral SThM image resolution of 25nm. In addition, the heat transfer through the tip to the sample was investigated using approach curves and used to determine the thermal conductivity of the 3nm thick HfO2 layer.