R. Heiderhoff

Quantitative thermal conductivity measurements with nanometre resolution

A quantitative thermal conductivity measurement technique, the method, is applied in a scanning thermal microscope (SThM) with a resistive probe for the determination of thermal properties with a high spatial resolution in the nanometre range. With this set-up the quantitative thermal conductivity of materials can be determined with a deviation of less than 2%. Using gold as the reference material, the local thermal conductivities of silver and a CVD diamond film have been measured with a spatial resolution of approximately 30 nm.

Straightforward Generation of Pillared, Microporous Graphene Frameworks for Use in Supercapacitors

Microporous, pillared graphene-based frameworks are generated in a simple functionalization/coupling procedure starting from reduced graphene oxide. They are used for the fabrication of high-performance supercapacitor devices.

Quantitative dynamic near-field microscopy of thermal conductivity

A new three-dimensional finite element method model of the conventional resistive thermal probe, usually employed within scanning thermal microscopy (SThM) has been developed. As a result, the line heat source characteristic of the bent thermal sensitive filament seems to permit the explanation of the experimental results within a certain frequency range. The verification of this line heat source characteristic of the thermal probe finally leads to the introduction of a general near-field condition of SThM, which considers the spatial and temporal characteristic of the heat source. Here, the contact area between the probe and the sample is not considered as the source but rather as the aperture between the heat source and the sample. In addition, the combination of this kind of thermal probe with the so-called 3ω-method has been justified for the quantitative determination of the local thermal conductivity. Moreover, the applicable sample thermal conductivity range has been expanded significantly by considering the varying heat flow into the sample within the conditional equation.

A universal scanning-probe-microscope-based hybrid system

A modularly equipped scanning probe microscope (SPM)-based hybrid system is developed, expanding application fields and performances of former instruments. The SPM head can easily be incorporated into the analysis chamber of most scanning electron microscopes (SEMs) and both systems can be handled by the electronics within one software simultaneously. Contact and non-contact scanning force microscope (SFM) measurements obtained inside an environmental SEM for the first time demonstrate resolutions of less than 0.2 nm under working conditions. Both probes can be used simultaneously either as actuators or sensors to deliberately modify and analyse sample properties as well as to characterize the probe interactions. This is the essential advantage of hybrid systems, besides the great number of well known complementary analyses which can be performed with each microscope at the same sample area. The use of an electron beam and a SFM tip as two independent electrical actuators and sensors is demonstrated to be exemplary. Interaction of the probes with each other is a new aspect, which is presented and discussed.

Plasmonically sensitized metal-oxide electron extraction layers for organic solar cells

ZnO and TiOx are commonly used as electron extraction layers (EELs) in organic solar cells (OSCs). A general phenomenon of OSCs incorporating these metal-oxides is the requirement to illuminate the devices with UV light in order to improve device characteristics. This may cause severe problems if UV to VIS down-conversion is applied or if the UV spectral range (λ < 400 nm) is blocked to achieve an improved device lifetime. In this work, silver nanoparticles (AgNP) are used to plasmonically sensitize metal-oxide based EELs in the vicinity (1–20 nm) of the metal-oxide/organic interface. We evidence that plasmonically sensitized metal-oxide layers facilitate electron extraction and afford well-behaved highly efficient OSCs, even without the typical requirement of UV exposure. It is shown that in the plasmonically sensitized metal-oxides the illumination with visible light lowers the WF due to desorption of previously ionosorbed oxygen, in analogy to the process found in neat metal oxides upon UV exposure, only. As underlying mechanism the transfer of hot holes from the metal to the oxide upon illumination with hν < Eg is verified. The general applicability of this concept to most common metal-oxides (e.g. TiOx and ZnO) in combination with different photoactive organic materials is demonstrated.

Scanning near-field acoustic study of ferroelectric BaTiO3ceramics

Ferroelectric materials have attracted attention in recent years because of the combination of these materials with integrated circuit technology. Although much effort has been made to develop new devices by the use of these materials, there is still much to do to understand the so-called domain effects at submicrometer or even in nanometer range. Although near-field acoustic techniques have achieved considerable progress in this respect recently, a complete understanding of physical effects in the near field of these techniques could not be completed solely by the use of one method, as both ferroelectric materials and near-field acoustic techniques are complicated in nature. A complementary study by two or more techniques will bring a new view to the analyses and characterization of ferroelectric materials and integrated devices. This paper will present complementary investigations on BaTiO3 ceramics by the use of two non-destructive scanning near-field acoustic microscopy techniques on the identical areas of the same sample. A general model for both near-field acoustic techniques on ferroelectric BaTiO3 is presented. Explanations of the results based on present theory will also be discussed in detail.

Photonic Nanostructures Patterned by Thermal Nanoimprint Directly into Organo-Metal Halide Perovskites

Photonic nanostructures are created in organo-metal halide perovskites by thermal nanoimprint lithography at a temperature of 100 °C. The imprinted layers are significantly smoothened compared to the initially rough, polycrystalline layers and the impact of surface defects is substantially mitigated upon imprint. As a case study, 2D photonic crystals are shown to afford lasing with ultralow lasing thresholds at room temperature.

Electron-beam-induced potentials in semiconductors: calculation and measurement with an SEM/SPM hybrid system

In this work electron-beam-induced potentials are analysed theoretically and experimentally for semiconductors. A theoretical model is developed to describe the surface potential distribution produced by an electron beam. The distribution of generated carriers is calculated using semiconductor equations. This distribution causes a local change in surface potential, which is derived with the help of quasi-Fermi energies. The potential distribution is simulated using the model developed and measured with a scanning probe microscope (SPM) built inside a scanning electron microscope (SEM), for different samples, for different beam excitations and for different cantilever voltages of SPM. In the end, some fields of application are shown where material properties can be determined using an SEM/SPM hybrid system.

Near-field detection cathodoluminescence investigations

To make near-field detection cathodoluminescence investigations with a resolution in the nanometre regime feasible, a scanning near-field optical microscope (SNOM) has been implemented in the chamber of a scanning electron microscope. Two different modes of operation which can yield complementary information on the specimen can be carried out in our set-up. In the first mode, the SNOM probe position remains unchanged while the primary electron beam is scanned in raster fashion over the sample surface. In the second mode, the sample is homogeneously irradiated by the electron beam while being raster scanned underneath the SNOM probe. Whereas in the first mode specimen properties such as diffusion lengths are accessible, the achievable spatial resolution in the second mode depends only on the size of the aperture at the apex of the SNOM probe. A spatial resolution of well below 50 nm has been achieved on bulk YAG specimens in the second mode using commercially available probes.

Stress Management in Thin-Film Gas-Permeation Barriers.

Gas diffusion barriers (GDB) are essential building blocks for the protection of sensitive materials or devices against ambient gases, like oxygen and moisture. In this work, we study the mechanics of GDBs processed by atomic layer deposition (ALD). We demonstrate that a wide range of ALD grown barrier layers carry intrinsic mechanical tensile stress in the range of 400-500 MPa. In the application of these GDBs on top of organic electronic devices, we derive a critical membrane force (σ · h)crit = 1200 GPaÅ (corresponding to a layer thickness of about 300 nm) for the onset of cracking and delamination. At the same time, we evidence that thicker GDBs would be more favorable for the efficient encapsulation of statistically occurring particle defects. Thus, to reduce the overall membrane force in this case to levels below (σ · h)crit, we introduce additional compressively strained layers, e.g., metals or SiNx. Thereby, highly robust GDBs are prepared on top of organic light emitting diodes, which do not crack/delaminate even under damp heat conditions 85 °C/85% rh.