Optical polarization of excitons and trions under continuous and pulsed excitation in single layers of WSe2.

The potential for valleytronic operation has stimulated much interest in studying polarized emission from transition metal dichalcogenides. In most studies, however, little regard is given to the character of laser excitation. We measure the circularly polarized photoluminescence of WSe2 monolayers as a function of excitation energy for both continuous-wave (cw) and pulsed laser excitation sources. Using cw excitation, the temperature dependence of the depolarization of the trion follows the same trend as that of the neutral exciton and involves collisional broadening. However, the polarization of the trion is nearly twice the polarization of the neutral exciton at low temperatures. When a pulsed laser with the same average fluence is used as the excitation source, the degrees of polarization become very similar, in stark contrast to the cw results. The difference in polarization behaviors is linked to the different amounts of energy deposited in the system during these measurements for similar average fluences. At a moderate fluence, pulsed excitation also has the potential to fundamentally alter the emission characteristics of WSe2.

Optical polarization and intervalley scattering in single layers of MoS2 and MoSe2.

Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as a function of the difference between the excitation energy and the A-exciton emission at the K-point of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phonon-assisted intervalley scattering causes depolarization. In both materials this leads to almost complete depolarization within ~100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.

Anomalous temperature-dependent spin-valley polarization in monolayer WS2.

Single layers of transition metal dichalcogenides (TMDs) are direct gap semiconductors with nondegenerate valley indices. An intriguing possibility for these materials is the use of their valley index as an alternate state variable. Several limitations to such a utility include strong intervalley scattering, as well as multiparticle interactions leading to multiple emission channels. We prepare single-layer WS2 films such that the photoluminescence is from either the neutral or charged exciton (trion). After excitation with circularly polarized light, the neutral exciton emission has zero polarization. However, the trion emission has a large polarization (28%) at room temperature. The trion emission also has a unique, non-monotonic temperature dependence that is a consequence of the multiparticle nature of the trion. This temperature dependence enables us to determine that intervalley scattering, electron-hole radiative recombination, and Auger processes are the dominant mechanisms at work in this system. Because this dependence involves trion systems, one can use gate voltages to modulate the polarization (or intensity) emitted from TMD structures.

Spin transport and Hanle effect in silicon nanowires using graphene tunnel barriers.

Spin-based devices offer non-volatile, scalable, low power and reprogrammable functionality for emerging device technologies. Here we fabricate nanoscale spintronic devices with ferromagnetic metal/single-layer graphene tunnel barriers used to generate spin accumulation and spin currents in a silicon nanowire transport channel. We report the first observation of spin precession via the Hanle effect in both local three-terminal and non-local spin-valve geometries, providing a direct measure of spin lifetimes and confirmation of spin accumulation and pure spin transport. The use of graphene as the tunnel barrier provides a low-resistance area product contact and clean magnetic switching characteristics, because it smoothly bridges the nanowire and minimizes complicated magnetic domains that otherwise compromise the magnetic behaviour. Utilizing intrinsic two-dimensional layers such as graphene or hexagonal boron nitride as tunnel contacts on nanowires offers many advantages over conventional materials deposited by vapour deposition, enabling a path to highly scaled electronic and spintronic devices.

Vascular management during live donor nephrectomy: an online survey among transplant surgeons.

In 2006, a survey from the American Society of Transplant Surgeons disclosed significant and sometimes fatal hemorrhagic events in live donor nephrectomies (LDN) related to failure of clips, leading to the contraindication of the Weck® Hem-o-lok® clip for control of the renal artery during LDN. A survey regarding vascular control techniques, their perceived safety ratings and their failures was sent to 645 European Society for Organ Transplantation members who profiled their profession as "surgeon" and selected "kidney" as organ type. Two hundred forty-three (41%) members responded, of whom 171 (63.3%) independently perform LDN. Their responses were analyzed. For arterial and venous vascular control, the GIA™ and TA™stapler are used most frequently, and were rated the safest. Of the 121 reported hemorrhagic events, slippage and dislodgement of clips occurred at least 58 times, while stapler malfunction occurred at least 40 times. One donor death from hemorrhage related to clip dysfunction was reported. Hemorrhagic complications of LDN with fatal and non-fatal outcomes still occur. Strikingly, many surgeons do not use the vascular closing technique that they consider most safe. Failure of non-transfixion techniques is associated with greater risks for the donor. Control of major vessels in LDN must employ transfixion techniques for optimal donor safety.

Chemical vapor sensing with monolayer MoS2.

Two-dimensional materials such as graphene show great potential for future nanoscale electronic devices. The high surface-to-volume ratio is a natural asset for applications such as chemical sensing, where perturbations to the surface resulting in charge redistribution are readily manifested in the transport characteristics. Here we show that single monolayer MoS(2) functions effectively as a chemical sensor, exhibiting highly selective reactivity to a range of analytes and providing sensitive transduction of transient surface physisorption events to the conductance of the monolayer channel. We find strong response upon exposure to triethylamine, a decomposition product of the V-series nerve gas agents. We discuss these results in the context of analyte/sensor interaction in which the analyte serves as either an electron donor or acceptor, producing a temporary charge perturbation of the sensor material. We find highly selective response to electron donors and little response to electron acceptors, consistent with the weak n-type character of our MoS(2). The MoS(2) sensor exhibits a much higher selectivity than carbon nanotube-based sensors.

Low-resistance spin injection into silicon using graphene tunnel barriers.

Spin manipulation in a semiconductor offers a new paradigm for device operation beyond Moore's law. Ferromagnetic metals are ideal contacts for spin injection and detection, but the intervening tunnel barrier required to accommodate the large difference in conductivity introduces defects, trapped charge and material interdiffusion, which severely compromise performance. Here, we show that single-layer graphene successfully circumvents the classic issue of conductivity mismatch between a metal and a semiconductor for electrical spin injection and detection, providing a highly uniform, chemically inert and thermally robust tunnel barrier. We demonstrate electrical generation and detection of spin accumulation in silicon above room temperature, and show that the contact resistance-area products are two to three orders of magnitude lower than those achieved with oxide tunnel barriers on silicon substrates with identical doping levels. Our results identify a new route to low resistance-area product spin-polarized contacts, a key requirement for semiconductor spintronic devices that rely on two-terminal magnetoresistance, including spin-based transistors, logic and memory.

Bilayer graphene grown on 4H-SiC (0001) step-free mesas.

We demonstrate the first successful growth of large-area (200 × 200 µm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.

Regulatory failure contributing to deaths of live kidney donors.

Hemorrhagic deaths of living kidney donors from failure of vascular clips used on the renal artery, first documented in 2006, have continued due to postoperative Hem-o-lok clip failure with sudden, massive bleeding. While the FDA issued a Class II recall of the Hem-o-lok clip for laparoscopic donor nephrectomies in 2006, two live kidney donors in the United States and one in India have since died. Compliance in timely reporting of deaths by the manufacturer and donor hospitals has not been enforced. Oversight agencies did not inform practitioners that donors died due to clip failures. A February 2011 survey disclosed that Hem-o-lok or other clips are still used by some surgeons as a sole means of arterial control in laparoscopic donor nephrectomy; thus, a practice with documented fatal outcomes persists. We conclude that systems failures by oversight-regulatory agencies in communication to active clinicians led, at least in part, to preventable deaths. Information which was disseminated was neither complete nor timely. A corrective plan, funded by oversight agencies and the Hem-o-lok manufacturer, is proposed. All surgeons operating on a living organ donor must select vascular control techniques that entail tissue transfixion and assure a safe operative recovery. The Hem-o-lok and other surgical clips must not be used to control the donor renal artery.

Clinics aren't Meant for Men': Sexual Health Care Access and Seeking Behaviours Among Men in Gauteng Province; South Africa

Men may be key players in the transmission of sexually transmitted infections (STI); and it is important that STI/HIV health services reach men. The objective of this study was to explore sexual health care access and seeking behaviours in men. This study used focus groups to examine sexual health care access and seeking behaviours in men 5 years after implementation of free antiretroviral therapy (ART) in the South African public sector. Six focus groups (N=58) were conducted with men ?18 years in an urban area of Gauteng province. Men were recruited from various locations throughout the community. Men reported several barriers and facilitators to the use of public and private clinics for sexual health services including HIV testing; and many men reported seeking care from traditional healers. Men often viewed public clinics as a place for women and reported experiences with some female nurses who were rude or judgmental of the men. Additionally; some men reported that they sought sexual health care services at public clinics; however; they were not given physical examinations by health care providers to diagnose their STI syndrome. Most men lacked knowledge about ART and avoided HIV testing because of fear of death or being abandoned by their families or friends. Study findings suggest that men still require better access to high-quality; non-judgmental sexual health care services. Future research is needed to determine the most effective method to increase men's access to sexual health care services

Errors in ABO labeling of deceased donor kidneys: case reports and approach to ensuring patient safety.

Patient safety in transplantation depends on accurate testing, transcription and transmission of the ABO types of the donor and recipient. Similar to 'near-miss' transfusion labeling errors, three cases of mislabeled ABO types on deceased donor kidney containers were recognized through a pretransplant verification process. Six steps to confirming the organ and ABO identification were developed to ensure safety of the patient and prevent liability for the transplant team and facility. In each case, rapid recognition and documentation of the error source, on site confirmation of the ABO type of the accompanying blood specimen, and full disclosure to the patient and family permitted safe transplantation and avoided the need to pursue a more conservative course that would have required discarding the organs. We advocate following these measures in determining whether to persevere with transplantation of a mislabeled organ.

End-stage renal disease in diabetic persons: is the pandemic subsiding?

Analysis of data compiled by the United States Renal Data System and the National Health Interview Survey as reported in the Centers for Disease Control and Prevention's Weekly Morbidity and Mortality Report indicates that between 1990 and 2002, there has been a sharp decline in incidence rate of the number of persons with diabetes who develop end-stage renal disease. Although it is comforting to practitioners to attribute this improvement to a widely advocated regimen of renoprotection, consisting of careful regulation of hypertensive blood pressure, improved glycemic control, and lifestyle modification, evidence for this causal relationship is appearing only now. There is need to clarify the source of this epidemiologic change that will lessen the projected burden on medical and socioeconomic resources in the immediate future.

The broader burden of end-stage renal disease on children and their families.

When a child has end-stage renal disease and requires dialysis, a heavy personal and financial toll can be extracted from the caregivers and the family. Tsai et al. have demonstrated an adverse effect on the psychosocial and socioeconomic well-being of caregivers of children on chronic peritoneal dialysis. These findings raise other questions and force us to think about support for the caregiver as well as the patient.

Payment for donor kidneys: pros and cons.

Continuous growth of the end stage renal disease population treated by dialysis, outpaces deceased donor kidneys available, lengthens the waiting time for a deceased donor transplant. As estimated by the United States Department of Health & Human Services: '17 people die each day waiting for transplants that can't take place because of the shortage of donated organs.' Strategies to expand the donor pool--public relations campaigns and Drivers' license designation--have been mainly unsuccessful. Although illegal in most nations, and viewed as unethical by professional medical organizations, the voluntary sale of purchased donor kidneys now accounts for thousands of black market transplants. The case for legalizing kidney purchase hinges on the key premise that individuals are entitled to control of their body parts even to the point of inducing risk of life. One approach to expanding the pool of kidney donors is to legalize payment of a fair market price of about 40,000 dollars to donors. Establishing a federal agency to manage marketing and purchase of donor kidneys in collaboration with the United Network for Organ Sharing might be financially self-sustaining as reduction in costs of dialysis balances the expense of payment to donors.

Improved survival in HIV-infected African-Americans with ESRD.

AIMS: To determine if there has been improvement in survival of HIV-infected patients with end-stage renal failure subsequent to widespread use of highly active antiretroviral therapy. METHODS: The United States Renal Data System is a national data system funded by the National Institute of Diabetes and Digestive and Kidney Disease with the Centers for Medicare and Medicaid. Using the United States Renal Data System Standard Analysis Files, we analyzed all African-American end-stage renal failure patients in the United States from 1990-2001. We compared survival rates for patients with HIV disease, sickle cell anemia, diabetes, and all other diagnoses for the time periods 1990-1994 and 1995-2001. The main outcome measure was one- and five-year survival in each cohort. RESULTS: One-year survival of African-American patients with end-stage renal disease and HIV increased from 46.6% during 1990-1994 to 65.1% during 1995-2001 (odds ratio 2.139). One-year survival decreased in the sickle cell group (odds ratio 0.595) and decreased slightly in the diabetic group (odds ratio 0.927) and all others (odds ratio 0.941). Five-year survival in the HIV group increased from 13.3% in 1990-1995 to 30.4% in 1995-2001 (odds ratio 2.847). There was no corresponding increase in survival for the sickle cell group (odds ratio 0.987), the diabetic group (odds ratio 1.06), or all others (odds ratio 1.137). CONCLUSIONS: We conclude that survival in African-American end-stage renal disease patients and HIV infection has substantially improved subsequent to introduction of highly active antiretroviral therapy. Our data support aggressive multi-drug treatment of end-stage renal failure patients with HIV infection.

Angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers used for the treatment of hypertension appear to be safe in the early posttransplant period.

Angiotensin II converting enzyme inhibitors (ACE) and angiotensin II receptor blockers (ARB) are frequently avoided as treatment for hypertension in the early posttransplant period (0 to 90 days) for fear of nephrotoxicity. The following retrospective study was designed to explore the safety of these drugs in the early posttransplant period and determine appropriate clinical criteria for starting these medications. The records of all adult renal transplants performed between January 1997 and December 2000 (N = 290) were reviewed. All patients started on ACE or ARB for treatment of hypertension within 90 days of their transplant were included in the analysis (n = 17). Controls (n = 19) were patients who were treated with a calcium channel blocker (CCB) for hypertension. Cases and controls were matched for gender and type of transplant, living or cadaver. Patients were considered to be enrolled whey they met the following criteria: hypertension, serum creatinine < or =3.0 mg/dL, or falling 1 mg/dL/d and serum potassium < or =5.5 mEq/L. Exclusion criteria were coexistent pancreas transplant, anaphylaxis to ACE, and use of ACE or ARB for treatment of posttransplant erythrocytosis. There were no differences between cases and controls in hemoglobin or serum potassium concentrations or calculated glomerular filtration rate at 3, 6, and 9 months. In renal transplant patients with reasonable allograft function, administration of ACE and ARB to treat hypertension in the early posttransplant period appears to be well tolerated and not associated with excess hyperkalemia, anemia, or acute renal dysfunction.

The evolving experience using everolimus in clinical transplantation.

Everolimus is a derivative of sirolimus, a macrocyclic lactone, originally isolated from Streptomyces hygroscopicus. Both everolimus and sirolimus have a similar mechanism of action, exerting potent inhibition of growth factor-induced proliferation of lymphocytes, as well as other hematopoietic and nonhematopoietic cells of mesenchymal origin. Each agent complexes with the FK506 binding protein 12 to inhibit cyclin dependent kinase(s), collectively termed the target of rapamycin (TOR), causing G1-S phase cell cycle arrest. Safety and efficacy have been documented in large-scale, blinded, randomized, international clinical renal and cardiac transplant trials. Everolimus is more hydrophilic, exhibits a shorter elimination half-life (approximately 30 hours), and demonstrates greater relative bioavailability compared to sirolimus. However, similar to the calcineurin inhibitors and sirolimus, everolimus is biotransformed by the cytochrome P450, 3A4 isozyme. Also similar to sirolimus, clinical experiences identified biologically relevant side effects including hyperlipidemia and exacerbation of cyclosporine (CsA)-associated nephrotoxicity. However, also similar to sirolimus, accumulating evidence suggests that the hyperlipidemia can be controlled and the CsA-associated renal effects appear reduced with a low incidence of acute rejection when everolimus is administered in combination with reduced CsA doses. The experience using everolimus in cardiac transplantation has also provided potentially important insights into the consequences of antiproliferative effects on vascular smooth muscle cells and fibroblasts where reduction in intimal expansion was identified by intravascular coronary ultrasound examination among those patients receiving everolimus. Therefore, available results suggest that the introduction of everolimus as the newest TOR inhibitor should enhance therapeutic options for immunosuppression after organ transplantation.

Incidence, timing and site of infections among pancreas transplant recipients.

The incidence, timing and site of infections among the different categories of pancreas transplant recipients were investigated. Patients were divided into three groups: pancreas transplant alone (PTA), pancreas after kidney transplant (PAK), or simultaneous pancreas and kidney (SPK) transplants. Length of follow-up, time to death, pancreas graft survival, incidence, timing and site of bacterial infections were noted. Our study showed that at least 75% of pancreas transplant recipients experienced at least one infection (range from 77.8% in the PTA group to 86.7% in the PAK group). The SPK group presented the highest rate of infections with 35.1 infections per 1000/patient-days. Symptomatic urinary tract infections were the most common cause of infection in all patients. The incidence of infections was higher during the first month after transplantation, except for the SPK transplant group, where infections occurred over a longer time period.

Sirolimus-based immunosuppression with reduce dose cyclosporine or tacrolimus after renal transplantation.

Sirolimus (SRL), a fermentation product of Streptomyces hygroscopicus, complexes with the FKBP12 to inhibit cyclin dependent kinase(s), collectively termed the target of rapamycin (TOR), causing G(1)-S phase cell cycle arrest. Safety and efficacy have been documented in clinical renal transplantation, but concerns were raised due to important biologically relevant side effects. Hyperlipidemia was identified, beginning with early clinical experiences, and the unexpected findings that SRL may exacerbate CsA associated nephrotoxicity was observed during the pivotal phase III studies. This report details results of our experience using SRL (target trough concentration, 10-15 ng/mL) with low dose CsA (target trough concentration, 50-100 ng/mL), seeking to determine whether this approach might provide effective immunosuppression while reducing associated nephrotoxicity. Among 121 renal transplant recipients, 62 received the SRL based regimen and 59 received MMF with all patients receiving CsA and prednisone. Similar to earlier clinical experiences, hematopoeitic abnormalities and hyperlipidemia were observed among patients who received SRL, and those abnormalities were readily controlled. However, unlike observations from the phase III SRL studies, renal function was not adversely affected. These findings support the growing body of evidence indicating that SRL based immunosuppression in combination low dose calcineurin inhibitors and corticosteroids is safe, efficacious, and without associated renal toxicity.

Aspergillus vertebral osteomyelitis after simultaneous kidney-pancreas transplantation.

Aspergillus osteomyelitis is a rare complication of invasive aspergillosis after organ transplantation. This is the report of a 46-year-old man who underwent a simultaneous pancreas and kidney transplantation, complicated by an Aspergillus osteomyelitis and diskitis of the lumbar spine. Prompt diagnosis with needle biopsy, followed by antifungal therapy using caspofungin, a new antifungal agent recommended for the treatment of refractory aspergillosis, in combination with amphotericin B and an early surgical intervention led to clinical resolution of the infection. Reported cases of spinal aspergillosis after transplantation are reviewed in terms of clinical presentation, risk factors, therapeutic options, and outcome.