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Mesoporous germanium (MP-Ge) has been predicted to play an important role in a wide range of potential applications. These porous Ge networks are characterized by physical and chemical properties that contrast with their bulk counterpart. For more than two decades, the methods that were used to synthesize these materials were extremely slow and yield non-uniform layers, which limit their applications. Here we demonstrate the synthesis of thick MP-Ge films with high growth rate (up to 300 nm/min) by using high-current density and high-frequency bipolar electrochemical etching. An optimized nucleation procedure is also reported. It enables a much better lateral homogeneity of the MP-Ge films. Following these results a refined electrochemical Ge etching model is proposed. The model is confirmed by Fourier Transform Infrared Spectroscopy and Cyclic Voltammetry measurements. Reflectivity measurements were also used to demonstrate that the effective refractive index of MP-Ge can be adjusted as a function of layer porosity. It has been reduced down to 1.76 due to the presence of the mesopores. Finally, a basic interferometric porous Ge sensor is demonstrated, a detection sensitivity of 711 nm/RIU was measured upon exposure to glycerol-water solutions. [ABSTRACT FROM AUTHOR]

The effect of the passivation to etching duration ratio on the bipolar electrochemical etching of highly p-doped germanium on its pore structures has been investigated. The final goal is the development of porous multi-layer stacks consisting of a closed growth template and separation layer for lift-off. In particular, changing the passivation duration allows different types of porous structures to be obtained. These range from a strong crystallographic orientation dependent dissolution to multi-layer structures consisting of "sponge" and "pine-tree" shapes. In this work, the degree of passivation was considered first and foremost since it is one way of controlling the formation of molecular hydrogen and surface atom protection. In addition, certain areas of the porous structure can be protected and, assuming a different space charge region width, selective etching is possible. Finally, by combining various successive processes, a porous multi-layer structure can be etched, including separation layers. In this work the most dominant reoccurring structures have been described using a model that contains the aforementioned assumptions. [ABSTRACT FROM AUTHOR]

The extended analysis of influence of the surface hydrogen passivation degree on morphology of mesoporous Ge (PGe) formed by Bipolar Electrochemical Etching technique is reported. It is shown that the high degree of the surface passivation results in formation of the isotropic sponge-like PGe layers, whereas at low degree of the surface passivation the crystallographic “pine-tree-like” pores are obtained. The intermediate passivation results in the “fish-bone” morphology. It should be noted that the last two morphologies were never obtained before and are unique to mesoporous Ge. The electrochemical reactions taking place on Ge during BEE process are identified by analyzing the evolution of the surface potential during the BEE process; the results were interpreted similarly to the cyclic voltametry data. Finally, a general electrochemical model describing the pore formation with different morphologies depending on the surface passivation degree is introduced. [ABSTRACT FROM AUTHOR]

Abstract: The formation of mesoporous Ge layers with different morphologies by bipolar electrochemical etching is reported for the first time. A detailed analysis of the effect of anodic/cathodic pulse duration on the mesoporous Ge formation is performed. It is shown that together with the applied current density and anodization time this parameter has the most important influence on porous Ge morphology. Six different types of porous morphologies were identified with the nanopore diameter being in the range from 5 to 15nm. Their crystalline nature has been confirmed by Raman spectroscopy. The diameters of the nanocrystallites of the porous Ge layers were extracted from Raman spectra by using the phonon-confinement model. Depending of the layer morphology their values varied between 4 and 10nm. A conceptual analysis of several aspects of the formation mechanisms of mesoporous Germanium based on the available information has allowed us to predict and create a new complex morphology. [Copyright &y& Elsevier]

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is a high-throughput analytical technique ideally suited for small-molecule detection from different bodily fluids (e.g., saliva, urine, and blood plasma). Many SALDI-MS substrates require complex fabrication processes and further surface modifications. Furthermore, some substrates show instability upon exposure to ambient conditions and need to be kept under special inert conditions. We have successfully optimized mesoporous germanium (meso-pGe) using bipolar electrochemical etching and efficiently applied meso-pGe as a SALDI-MS substrate for the detection of illicit drugs such as in the context of workplace, roadside, and antiaddictive drug compliance. Argon plasma treatment improved the meso-pGe efficiency as a SALDI-MS substrate and eliminated the need for surface functionalization. The resulting substrate showed a precise surface geometry tuning by altering the etching parameters, and an outstanding performance for illicit drug detection with a limit of detection in Milli-Q water of 1.7 ng/mL and in spiked saliva as low as 5.3 ng/mL for cocaine. The meso-pGe substrate had a demonstrated stability over 56 days stored in ambient conditions. This proof-of-principle study demonstrates that meso-pGe can be reproducibly fabricated and applied as an analytical SALDI-MS substrate which opens the door for further analytical and forensic high-throughput applications.