Instability and Spontaneous Reconstruction of Few-Monolayer Thick GaN Graphitic Structures.

Two-dimensional (2D) semiconductors are a very hot topic in solid state science and technology. In addition to van der Waals solids that can be easily formed into 2D layers, it was argued that single layers of nominally 3D tetrahedrally bonded semiconductors, such as GaN or ZnO, also become flat in the monolayer limit; the planar structure was also proposed for few-layers of such materials. In this work, using first-principles calculations, we demonstrate that contrary to the existing consensus the graphitic structure of few-layer GaN is unstable and spontaneously reconstructs into a structure that remains hexagonal in plane but with covalent interlayer bonds that form alternating octagonal and square (8|4 Haeckelite) rings with pronounced in-plane anisotropy. Of special interest is the transformation of the band gap from indirect in planar GaN toward direct in the Haeckelite phase, making Haeckelite few-layer GaN an appealing material for flexible nano-optoelectronics.

Understanding Phase-Change Memory Alloys from a Chemical Perspective.

Phase-change memories (PCM) are associated with reversible ultra-fast low-energy crystal-to-amorphous switching in GeTe-based alloys co-existing with the high stability of the two phases at ambient temperature, a unique property that has been recently explained by the high fragility of the glass-forming liquid phase, where the activation barrier for crystallisation drastically increases as the temperature decreases from the glass-transition to room temperature. At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown. In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons. Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

Study of band inversion in the PbxSn1-xTe class of topological crystalline insulators using x-ray absorption spectroscopy.

Pb(x)Sn(1-x)Te and Pb(x)Sn(1-x)Se crystals belong to the class of topological crystalline insulators where topological protection is achieved due to crystal symmetry rather than time-reversal symmetry. In this work, we make use of selection rules in the x-ray absorption process to experimentally detect band inversion along the PbTe(Se)-SnTe(Se) tie-lines. The observed significant change in the ratio of intensities of L1 and L3 transitions along the tie-line demonstrates that x-ray absorption can be a useful tool to study band inversion in topological insulators.

Transport properties in a Sb-Te binary topological-insulator system.

Sb-Te layers having various compositions between Sb2Te3 and Sb2Te are grown using molecular beam epitaxy. The structural and electrical properties of the layers change gradually with composition but exhibit a discontinuity involving a bistability. The holes in the layers are generated by Sb bilayers intercalated between Sb2Te3 quintuple layers and their mobility is governed by the scattering from the parent acceptors. Magnetoresistance for compositions around SbTe is linear, for which the reduction of the parabolic component due to low mobility is crucial. Density functional calculations predict Sb2Te3 and SbTe to be topological insulators (TIs) resembling Bi2Se3 and Bi2Te3, respectively. The prefactor of the weak antilocalization effect is α =- 1 regardless of the composition. The Sb-Te system is thus a family of TIs possessing undisturbed surface states for which the location of the Dirac point with respect to the bulk band gap is adjustable.

Interfacial phase-change memory.

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

Distortion-triggered loss of long-range order in solids with bonding energy hierarchy.

An amorphous-to-crystal transition in phase-change materials like Ge-Sb-Te is widely used for data storage. The basic principle is to take advantage of the property contrast between the crystalline and amorphous states to encode information; amorphization is believed to be caused by melting the materials with an intense laser or electrical pulse and subsequently quenching the melt. Here, we demonstrate that distortions in the crystalline phase may trigger a collapse of long-range order, generating the amorphous phase without going through the liquid state. We further show that the principal change in optical properties occurs during the distortion of the still crystalline structure, upsetting yet another commonly held belief that attributes the change in properties to the loss of long-range order. Furthermore, our results suggest a way to lower energy consumption by condensing phase change inducing energy into shorter pulses or through the use of coherent phonon excitation.

Toward the ultimate limit of phase change in Ge(2)Sb(2)Te(5).

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the material's ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

Initial structure memory of pressure-induced changes in the phase-change memory alloy Ge2Sb2Te5.

We demonstrate that while the metastable face-centered cubic (fcc) phase of Ge2Sb2Te5 becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. Upon higher compression, a body-centered cubic phase is obtained in both cases around 30 GPa. Upon decompression, the amorphous phase is retained for the starting fcc phase while the initial structure is recovered for the starting trigonal phase. We argue that the presence of vacancies and associated subsequent large atomic displacements lead to nanoscale phase separation and loss of initial structure memory in the fcc staring phase of Ge2Sb2Te5.

Plasmonic devices with controllable resonances--an avenue towards high-speed and mass fabrication of optical meta-materials.

We report on the fabrication of plasmonic devices with adjustable resonances in the visible portion employing a thermal lithography method. The genuine approach enables the fabrication of nanostructured pattern at a spatial resolution comparable to other nanofabrication techniques, but at significantly larger speeds and over extended spatial domains. The fabricated structures consisted in periodically arranged nanoholes in a silver thin film and supported localized plasmon resonance (LPR) in the vicinity of 370 nm. Results from measured spectral properties were in good agreement with simulations based on rigorous diffraction theory. The method was evaluated towards a potential application to realize large-scale meta-materials with effective negative refractive index in the visible.

Biomolecular sensors utilizing waveguide modes excited by evanescent fields.

Properties of evanescent-field-coupled waveguide-mode sensors consisting of a multi-layer structure made of a SiO(2) waveguide, a thin metal layer (Au, Cu, W or Ti), and a high refractive index glass substrate illuminated under the Kretschmann configuration have been theoretically and experimentally investigated. In all cases, reflectivity changes attributed to streptavidin combining to biotinyl groups were observed in close spectral vicinity of the waveguide resonances. The sensors with the Au and the Cu layers show superior sensitivity as compared to those with the W and the Ti layers, whereas the W and Ti layers show better thermal and chemical stability. The results indicate that the materials of thin metal layers should be chosen in accordance with the purpose of sensors and/or environment in which the sensors are used.

Pressure-induced site-selective disordering of Ge2Sb2Te5: a new insight into phase-change optical recording.

We demonstrate that , the material of choice in phase-change optical recording (such as DVD-RAM), can be rendered amorphous by the application of hydrostatic pressure. It is argued that this structural change is due to a very strong second-nearest-neighbor Te-Te interaction that determines the long-range order in the metastable cubic phase of and also to the presence of vacancies. This newly discovered phenomenon suggests that pressure is an important factor for the formation of the amorphous phase which opens new insight into the mechanism of phase-change optical recording.

Thermal-induced optical properties of a PdOx mask layer in an optical data storage system with a superresolution near-field structure.

Optical-thermal and thermal-optical properties of a PdOx mask layer in a system with a superresolution near-field structure are investigated with a Z-scan technique and a heating experiment. The high photothermal stability of the PdOx mask is shown, and the reversible limit of the PdOx mask layer and a weak switch effect are revealed. The PdOx decomposition, which results in a bubble with Pd particles, is confirmed, and the laser-induced physical and chemical mechanisms in the PdOx mask layer are clarified and discussed. Our microscopic studies and heating analysis are consistent with the Z-scan results. The PdOx mask sample is also compared briefly with a PtO2 mask layer that has the same structure.

MRimaging findings after ventricular puncture in patients with SAH.

OBJECT: Using magnetic resonance (MR) imaging, we studied brain injury from ventricular puncture performed during craniotomy in the acute stage of subarachnoid hemorrhage (SAH). METHODS: 80 patients underwent craniotomy for aneurysm obliteration within 48 hr after SAH, ventricular puncture for drainage of cerebrospinal fluid (CSF) was performed to reduce intracranial pressure. MR imaging was performed within 3 days following surgery to measure the size of the lesion, and was repeated on postoperative days 14 and 30. CONCLUSIONS: Of the 80 patients with ventricular puncture preceding craniotomy, 65 (81%) showed MR evidence of brain injury from the puncture. Overall, 149 lesions were detected. According to coronal images, cortical injuries (54 cases), penetrating injury to tracts along the ventricular tube (55 cases), caudate injury (25 cases), and corpus callosum injury (15 cases). Brain injuries from ventricular puncture did not correlate significantly to patient outcome. While ventricular puncture and drainage of CSF can readily be performed to decrease brain volume at the time of craniotomy in acute-stage SAH, neurosurgeons should be aware of a surprisingly high incidence of brain injury complicating puncture.

Superresolution optical disk with a thermoreversible organic thin film.

Recording and retrieving small marks far beyond the optical diffraction limit in a high-speed rotating phase-change optical disk have been investigated by use of a thermoreversible organic thin film as a superresolution mask layer. The organic thin film exhibited significant thermoreversibility and rapid response on laser irradiation. Recorded marks as small as 120 nm in length could be detected by a dynamic disk tester with a laser wavelength of 635 nm and a numerical aperture of 0.6.

Asymptomatic versus symptomatic infarcts from vasospasm in patients with subarachnoid hemorrhage: serial magnetic resonance imaging.

OBJECTIVE: By use of serial magnetic resonance imaging (MRI), we prospectively investigated the incidence of and the risk factors associated with infarction caused by vasospasm with or without a delayed ischemic neurological deficit (DIND) in patients with subarachnoid hemorrhage (SAH). METHODS: In 125 patients who underwent surgery for early aneurysms, postoperative MRI scans were obtained at four time points. We defined an infarct from vasospasm as a new lesion not present on the initial MRI within 3 days after SAH and therefore not attributable to primary brain damage or surgical complications. RESULTS: Overall, symptoms of infarction (i.e., DIND) occurred in 38% of patients (48 of 125); DIND with a new infarct on MRI was evident in 34% (43 patients), whereas 4% (5 patients) showed no new lesion but had a DIND. However, 29 patients (23%) showed a new infarct but no DIND on MRI studies (asymptomatic infarction). Asymptomatic ischemic lesions due to vasospasm tended to involve noneloquent brain areas in the territory of intraparenchymal perforators. Multivariate analysis identified variables associated with symptomatic infarction to be of poor SAH grade, advanced age of the patient, angiographic findings of vasospasm, multiple cortical infarcts on MRI studies consistent with vasospasm, and chronic hydrocephalus. CONCLUSION: Analysis of the data confirmed the occurrence of asymptomatic infarcts due to vasospasm. These infarcts often developed in noneloquent areas representing perforator territory. MRI investigation of vasospastic lesions referable to intraparenchymal vessels such as perforators complements the study of extraparenchymal major vessel vasospasm in patients with SAH by computed tomographic angiography.

Efficacy of neuroendoscopic procedures in minimally invasive preferential management of pineal region tumors: a prospective study.

OBJECT: This prospective study is based on a consecutive series of 20 patients with pineal region tumors who underwent minimally invasive preferential management. The purpose of this report is to discuss the role of neuroendoscopic procedures in the management of pineal region tumors. METHODS: If the tumor markers alpha-fetoprotein and human chorionic gonadotropin were not detected in serum and there was significant ventricular dilation visualized on neuroimages, neuroendoscopic surgery was first applied for tumor debulking with tissue diagnosis and gross morphological analysis of the tumor and the intraventricular structures, followed by third ventriculostomy. Subsequent procedures were determined on the basis of verified individual tumors. For treatment of germinomas and pineoblastomas, if no tumor dissemination was confirmed by pre-, intra-, or postoperative findings, stereotactic radiotherapy or radiosurgery was performed after one course of chemotherapy with the ICE regimen (isofomid, cisplatin, and etoposide) and followed by two additional courses of chemotherapy. For treatment of malignant germ cell tumors, after extensive surgery, adjuvant chemotherapy with the ICE regimen was performed in three courses in all cases. Then radiotherapy was started using various methods, depending on the evidence of tumor dissemination. For treatment of teratomatous and neuroectodermal tumors other than pineoblastomas, extensive surgical removal was performed. As for adjuvant therapy, if the tumor was a low-grade glioma or if the patient was younger than 5 years of age, postoperative treatment did not include radiotherapy. If the tumor was a malignant teratoma or high-grade glioma, conventional focal radiotherapy was performed, followed by chemotherapy with ICE for 1 year. All but two treated patients had ventriculomegaly. Neuroendoscopic procedures were performed in six of 15 treated patients. Neuroendoscopic biopsy with tumor debulking offered enough material for tissue diagnosis, including immunohistochemical analysis and, in one case, revealed evidence of tumor dissemination undetectable on neuroimaging. With one exception, no shunt was required in any patient undergoing endoscopic third ventriculostomy. Stereotactic radiotherapy was performed in indicated cases. Favorable therapeutic outcomes were obtained in all cases of germinoma and pineoblastoma, with follow-up periods ranging from 24 months to 6.5 years. CONCLUSIONS: Our minimally invasive preferential regimen clarified the precise indication for neuroendoscopic procedures, and the majority of our patients with dilated ventricles and no evidence of tumor markers were treated satisfactorily with effective neuroendoscopic procedures as the initial procedure, avoiding unnecessary craniotomy and radiotherapy and promising excellent therapeutic outcomes. The treatment for malignant pineal region tumors remains a subject for further study.

Results of early surgical evacuation of packed intraventricular hemorrhage from aneurysm rupture in patients with poor-grade subarachnoid hemorrhage.

OBJECT: The goal of this study was to evaluate the results of early surgical evacuation of "packed" intraventricular hemorrhage (IVH) in patients with poor-grade subarachnoid hemorrhage (SAH). METHODS: The authors performed surgery within 24 hours after onset of SAH, identified on neuroimaging as a cast distending the ventricular system, in 74 patients with poor-grade SAH (World Federation of Neurosurgical Societies Grades IV and V) without intracerebral hemorrhage. Eighteen of these patients had packed IVH; in these patients the intraventricular clots were extensively evacuated via frontal corticotomy performed under microscopic view. CONCLUSIONS: Overall, 42% of the 74 patients undergoing craniotomy in the acute stage had favorable outcomes, whereas 30% died. Using multivariate analysis, variables significantly associated with favorable outcome in patients with poor-grade SAH included absence of a packed intraventricular clot on computerized tomography scanning; absence of a history of cardiac disease; and a Glasgow Coma Scale score of 11 or 12. None of the 18 patients who had packed IVH had favorable outcomes and seven of these died. In six recently treated patients with packed IVH, which was examined using fluid-attenuated inversion recovery imaging, extensive periventricular brain damage was found both immediately after surgery and during the chronic stage. Accordingly, the authors believe that irreversible periventricular brain damage is already complete immediately after packed IVH occurs.