Nano-LED induced chemical reactions for structuring processes

We present a structuring technique based on the initialization of chemical reactions by an array of nano-LEDs which is used in the near-field as well as in the far-field regime. In the near-field regime, we demonstrate first results with the nano-LED array for lithography using the photoresist DiazoNaphthoQuinone-(DNQ)-sulfonate for the fabrication of holes in the resist down to ∼75 nanometres in diameter. In contrast, the nano-LEDs can also be employed in the far-field regime to expose thin films of the monomer bisphenol A-glycidyl methacrylate (Bis-GMA) and to initialize polymerization locally. Photosensitive films were patterned and spherical cone-shaped three dimensional objects with diameters ranging from ∼480 nm up to 20 micrometres were obtained. The modification in the material as a result of the photochemical reaction induced i.e. by polymerization was confirmed by Raman spectroscopy. This structuring maskless technique has the potential to induce substantial changes in photosensitive molecules and to produce the desired structures from the tens of microns down to the nanometre scale.

TaS3 Nanofibers: Layered Trichalcogenide for High-Performance Electronic and Sensing Devices

Layered materials, like transition metal dichalcogenides, exhibit broad spectra with outstanding properties with huge application potential, whereas another group of related materials, layered transition metal trichalcogenides, remains unexplored. Here, we show the broad application potential of this interesting structural type of layered tantalum trisulfide prepared in a form of nanofibers. This material shows tailorable attractive electronic properties dependent on the tensile strain applied to it. Structure of this so-called orthorhombic phase of TaS₃ grown in a form of long nanofibers has been solved and refined. Taking advantage of these capabilities, we demonstrate a highly specific impedimetric NO gas sensor based on TaS₃ nanofibers as well as construction of photodetectors with excellent responsivity and field-effect transistors. Various flexible substrates were used for the construction of a NO gas sensor. Such a device exhibits a low limit of detection of 0.48 ppb, well under the allowed value set by environmental agencies for NO_x (50 ppb). Moreover, this NO gas sensor also showed excellent selectivity in the presence of common interferences formed during fuel combustion. TaS₃ nanofibers produced in large scale exhibited excellent broad application potential for various types of devices covering nanoelectronic, optoelectronic, and gas-sensing applications.

Tuning of fluorine content in graphene: towards large-scale production of stoichiometric fluorographene

The availability of well-defined modified graphene derivatives such as fluorographene, graphane, thiographene or hydroxygraphene is of pivotal importance for tuning the resulting material properties in numerous potential applications. A series of fluorinated graphene with various contents of fluorine was synthesized by a simple fluorination procedure in an autoclave with a nitrogen/fluorine atmosphere at different exposure times and temperatures. To investigate the composition, structure and properties all samples were characterized in detail by a number of analytical techniques such as SEM, XRD, EDS, AFM, STEM, combustible elemental analysis, STA, XPS, Raman spectroscopy, UV-VIS spectroscopy and cyclic voltammetry. The fully fluorinated graphene with the overall stoichiometry C1F1.05 had a bright white color indicating a significant change of band-gap. In comparison to other samples such a high concentration of fluorine led to the occurrence of exotic thermal behavior, strong luminescence in the visible spectral region and also the unique super-hydrophobic behavior observed on the material surface. The described tunable fluorination should pave the way to fluorographene based devices with tailored properties.

Insight into the mechanism of the thermal reduction of graphite oxide: deuterium-labeled graphite oxide is the key

For the past decade, researchers have been trying to understand the mechanism of the thermal reduction of graphite oxide. Because deuterium is widely used as a marker in various organic reactions, we wondered if deuterium-labeled graphite oxide could be the key to fully understand this mechanism. Graphite oxides were prepared by the Hofmann, Hummers, Staudenmaier, and Brodie methods, and a deuterium-labeled analogue was synthesized by the Hofmann method. All graphite oxides were analyzed not only using the traditional techniques but also by gas chromatography-mass spectrometry (GC-MS) during exfoliation in hydrogen and nitrogen atmospheres. GC-MS enabled us to compare differences between the chemical compositions of the organic exfoliation products formed during the thermal reduction of these graphite oxides. Nuclear analytical methods (Rutherford backscattering spectroscopy, elastic recoil detection analysis) were used to calculate the concentrations of light elements, including the ratio of hydrogen to deuterium. Combining all of these results we were able to determine graphite oxide's thermal reduction mechanism. Carbon dioxide, carbon monoxide, and water are formed from the thermal reduction of graphite oxide. This process is also accompanied by various radical reactions that lead to the formation of a large amount of carcinogenic volatile organic compounds, and this will have major safety implications for the mass production of graphene.

Nano-LED array fabrication suitable for future single photon lithography

We report on an alternative illumination concept for a future lithography based on single-photon emitters and important technological steps towards its implementation. Nano light-emitting diodes (LEDs) are chosen as the photon emitters. First, the development of their fabrication and their integration technology is presented, then their optical characteristics assessed. Last, size-controlled nano-LEDs, well positioned in an array, are electrically driven and utilized for illumination. Nanostructures are lithographically formed, demonstrating the feasibility of the approach. The potential of single-photon lithography to reach the ultimate scale limits in mass production is discussed.

Evolution and characteristics of GaN nanowires produced via maskless reactive ion etching

The formation of nanowires (NWs) by reactive ion etching (RIE) of maskless GaN layers was investigated. The morphological, structural and optical characteristics of the NWs were studied and compared to those of the layer they evolve from. It is shown that the NWs are the result of a defect selective etching process. The evolution of density and length with etching time is discussed. Densely packed NWs with a length of more than 1 μm and a diameter of ∼60 nm were obtained by RIE of a ∼2.5 μm thick GaN layer. The NWs are predominantly free of threading dislocations and show an improvement of optical properties compared to their layer counterpart. The production of NWs via a top down process on non-masked group III-nitride layers is assessed to be very promising for photovoltaic applications.

Vertically integrated (Ga, In)N nanostructures for future single photon emitters operating in the telecommunication wavelength range

Important technological steps are discussed and realized for future room-temperature operation of III-nitride single photon emitters. First, the growth technology of positioned single pyramidal InN nanostructures capped by Mg-doped GaN is presented. The optimization of their optical characteristics towards narrowband emission in the telecommunication wavelength range is demonstrated. In addition, a device concept and technology was developed so that the nanostructures became singularly addressable. It was found that the nanopyramids emit in the telecommunication wavelength range if their size is chosen appropriately. A p-GaN contacting layer was successfully produced as a cap to the InN pyramids and the top p-contact was achievable using an intrinsically conductive polymer PEDOT:PSS, allowing a 25% increase in light transmittance compared to standard Ni/Au contact technology. Single nanopyramids were successfully integrated into a high-frequency device layout. These decisive technology steps provide a promising route to electrically driven and room-temperature operating InN based single photon emitters in the telecommunication wavelength range.

Molecular properties of liquid crystals in the terahertz frequency range

We report on a first experimental study of the molecular properties of nematic liquid crystals in the terahertz range. In the beginning, we extract the frequency and temperature dependent refractive index and absorption coefficient of the cyanobiphenyls 5CB, 6CB and 7CB from terahertz time domain spectroscopy measurements and investigate the impact of the alkyl chain length on the macroscopic liquid crystal characteristics, focusing especially on the pronounced odd and even effect. Next, we deduce the principle polarizabilities and the order parameter S by applying Vuks' approximation and Haller's approach. On this basis, we calculate the main polarizabilities along the longitudinal and transverse axis and link the observed terahertz properties to the molecular structure of the liquid crystals.

Terahertz birefringence for orientation analysis

A terahertz time-domain spectrometer is employed to study different birefringent samples. We develop a method based on the temporal waveform and the impulse response of a sample to map the anisotropy of their inner structure. To validate our algorithm, we study the polarization-affecting structure of various classes of materials such as crystals, plastics, and natural products. Among all samples we observe the largest birefringence for a rutile crystal with Deltan=3.3 at 1 THz.

Impact of the contact metallization on the performance of photoconductive THz antennas

Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. Here, we present the first systematic comparison between these two metallization types. A series of antennas of both kinds is excited by femtosecond laser pulses and by the emission from two diode lasers, i.e. we test the structures as pulsed THz emitters and as photomixers. In both cases, coherent and incoherent detection schemes are employed. We find that the power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model we conclude that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.

Continuous wave terahertz spectrometer as a noncontact thickness measuring device

We present a low cost terahertz (THz) spectrometer with coherent detection based on two simple and robust dipole antennas driven by two laser diodes. The spectrometer covers frequencies up to 1 THz, with a peak signal-to-noise ratio exceeding 40 dB for a lock-in integration time of 30 ms. We demonstrate that the thickness profile of a sample can be reconstructed from an acquired THz image.

Femtosecond response of a free-standing LT-GaAs photoconductive switch

We present a novel, free-standing low-temperature GaAs (LT-GaAs) photoconductive switch and demonstrate its femtosecond performance. A 1-microm-thick layer of a single-crystal LT-GaAs was patterned into 5-10-microm-wide and 15-30-microm-long bars, separated from their GaAs substrate and, subsequently, placed across gold coplanar transmission lines deposited on a Si substrate, forming a photoconductive switch. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an electro-optic sampling system. Using 810-nm optical radiation, we recorded an electrical transient as short as 360 fs (1.25 THz, 3-dB bandwidth) and established that the photo-carrier lifetime in our LT-GaAs was 150 fs. Our free-standing devices exhibited quantum efficiency of the order of approximately 7%, and their photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the excitation power, below a well-defined saturation threshold.

Scaphoid nonunion

Scaphoid nonunion is common, but the exact pathophysiology of this complication is unclear. Explanations include lack of treatment, poor initial treatment, delay in diagnosis, synovial fluid dynamics, precarious vascularity, fracture displacement, and carpal instability. Currently, the diagnosis is best confirmed by classic changes on plain radiographs, instability testing, arthrography, and arthroscopy in selected cases. Nine carpal bones are not benign. The natural history of scaphoid nonunion is one of progressive arthritis. Attempts at obtaining bony healing are therefore recommended. In treating established nonunions without arthritis, the Russe bone graft technique is the mainstay of treatment. A union rate of 90 per cent is to be expected. Electrical stimulation is an alternative when there is no synovial pseudarthrosis or scaphoid collapse deformity, or if a previous Russe graft has failed. If a significant humpback scaphoid or collapse deformity is present, internal fixation with the Herbert screw and scaphoid reconstruction with a bone graft are our choices. Healing rates are less than those with the Russe graft, but one may achieve improved motion of the wrist and earlier return to function. When scaphoid nonunion is accompanied by degenerative arthritis, salvage procedures are recommended. Radial styloidectomy is a simple procedure that will preserve motion and buy time. Soft tissue interposition with excision of a small proximal pole is useful, particularly if no collapse deformity is present. Silicone replacement alone has fallen into increasing disfavor because of the high incidence of subluxation and silicone synovitis. Combining silicone replacement with intercarpal fusion (the SLAC procedure) may lessen these complications. Proximal row carpectomy is another procedure that may preserve motion, though often at the expense of weakness, particularly in the younger patient requiring significant grip strength. In these cases, standard wrist fusion seems the most predictable alternative.

Spinal neuroarthropathy after traumatic paraplegia

Spinal neuroarthropathy is a little-known complication of traumatic paraplegia. Four cases of this syndrome are described, with emphasis on the characteristic radiographic findings of severe juxta-articular bone destruction, dense appositional new bone formation, large osteophytosis, and soft-tissue bony debris. The factors predisposing patients to develop a neuropathic joint are diminished pain and proprioceptive sensations with maintained mobility. When a paraplegic patient transfers in or out of a wheelchair or moves his upper torso, he exerts force on an insensate spine. Repeated trauma increases joint mobility beyond the normal limits, and this leads to further damage, with the process culminating in severe instability and bone destruction. The other causes of neuropathic joints in the spine--tertiary syphilis, syringomyelia, and diabetes--must be ruled out on clinical grounds. Neuropathic changes in the spine are often silent, delaying treatment, or may be mistaken for infection or degenerative disease. Their true prevalence is difficult to determine, but the possibility should be considered in paraplegic patients with the characteristic radiographic findings.