Excitation-Dependent Anisotropic Raman Response of Atomically Thin Pentagonal PdSe2.

The group-10 noble-metal dichalcogenides have recently emerged as a promising group of two-dimensional materials due to their unique crystal structures and fascinating physical properties. In this work, the resonance enhancement of the interlayer breathing mode (B1) and intralayer Ag 1 and Ag 3 modes in atomically thin pentagonal PdSe2 were studied using angle-resolved polarized Raman spectroscopy with 13 excitation wavelengths. Under the excitation energies of 2.33, 2.38, and 2.41 eV, the Raman intensities of both the low-frequency breathing mode B1 and high-frequency mode Ag 1 of all the thicknesses are the strongest when the incident polarization is parallel to the a axis of PdSe2, serving as a fast identification of the crystal orientation of few-layer PdSe2. We demonstrated that the intensities of B1, Ag 1, and Ag 3 modes are the strongest with the excitation energies between 2.18 and 2.38 eV when the incident polarization is parallel to PdSe2 a axis, which arises from the resonance enhancement caused by the absorption. Our investigation reveals the underlying interplay of the anisotropic electron-phonon and electron-photon interactions in the Raman scattering process of atomically thin PdSe2. It paves the way for future applications on PdSe2-based optoelectronics.

Surfactant-Mediated Growth and Patterning of Atomically Thin Transition Metal Dichalcogenides.

The role of additives in facilitating the growth of conventional semiconducting thin films is well-established. Apparently, their presence is also decisive in the growth of two-dimensional transition metal dichalcogenides (TMDs), yet their role remains ambiguous. In this work, we show that the use of sodium bromide enables synthesis of TMD monolayers via a surfactant-mediated growth mechanism, without introducing liquefaction of metal oxide precursors. We discovered that sodium ions provided by sodium bromide chemically passivate edges of growing molybdenum disulfide crystals, relaxing in-plane strains to suppress 3D islanding and promote monolayer growth. To exploit this growth model, molybdenum disulfide monolayers were directly grown into desired patterns using predeposited sodium bromide as a removable template. The surfactant-mediated growth not only extends the families of metal oxide precursors but also offers a way for lithography-free patterning of TMD monolayers on various surfaces to facilitate fabrication of atomically thin electronic devices.

Two-Dimensional Palladium Diselenide with Strong In-Plane Optical Anisotropy and High Mobility Grown by Chemical Vapor Deposition.

Two-dimensional (2D) palladium diselenide (PdSe2 ) has strong interlayer coupling and a puckered pentagonal structure, leading to remarkable layer-dependent electronic structures and highly anisotropic in-plane optical and electronic properties. However, the lack of high-quality, 2D PdSe2 crystals grown by bottom-up approaches limits the study of their exotic properties and practical applications. In this work, chemical vapor deposition growth of highly crystalline few-layer (≥2 layers) PdSe2 crystals on various substrates is reported. The high quality of the PdSe2 crystals is confirmed by low-frequency Raman spectroscopy, scanning transmission electron microscopy, and electrical characterization. In addition, strong in-plane optical anisotropy is demonstrated via polarized Raman spectroscopy and second-harmonic generation maps of the PdSe2 flakes. A theoretical model based on kinetic Wulff construction theory and density functional theory calculations is developed and described the observed evolution of "square-like" shaped PdSe2 crystals into rhombus due to the higher nucleation barriers for stable attachment on the (1,1) and (1,-1) edges, which results in their slower growth rates. Few-layer PdSe2 field-effect transistors reveal tunable ambipolar charge carrier conduction with an electron mobility up to ≈294 cm2 V-1 s-1 , which is comparable to that of exfoliated PdSe2 , indicating the promise of this anisotropic 2D material for electronics.

Atomically Precise PdSe2 Pentagonal Nanoribbons.

We report atomically precise pentagonal PdSe2 nanoribbons (PNRs) fabricated on a pristine PdSe2 substrate with a hybrid method of top-down and bottom-up processes. The PNRs form a uniform array of dimer structure with a width of 2.4 nm and length of more than 200 nm. In situ four-probe scanning tunneling microscopy (STM) reveals metallic behavior of PNRs with ballistic transport for at least 20 nm in length. Density functional theory calculations produce a semiconducting density of states of isolated PNRs and find that the band gap narrows and disappears quickly once considering coupling between PNR stacking layers or interaction with the PdSe2 substrate. The coupling of PNRs is further corroborated by Raman spectroscopy and field-effect transistor measurements. The facile method of fabricating atomically precise PNRs offers an air-stable functional material for dimensional control.

Defect-Mediated Phase Transformation in Anisotropic Two-Dimensional PdSe2 Crystals for Seamless Electrical Contacts.

The failure to achieve stable Ohmic contacts in two-dimensional material devices currently limits their promised performance and integration. Here we demonstrate that a phase transformation in a region of a layered semiconductor, PdSe2, can form a contiguous metallic Pd17Se15 phase, leading to the formation of seamless Ohmic contacts for field-effect transistors. This phase transition is driven by defects created by exposure to an argon plasma. Cross-sectional scanning transmission electron microscopy is combined with theoretical calculations to elucidate how plasma-induced Se vacancies mediate the phase transformation. The resulting Pd17Se15 phase is stable and shares the same native chemical bonds with the original PdSe2 phase, thereby forming an atomically sharp Pd17Se15/PdSe2 interface. These Pd17Se15 contacts exhibit a low contact resistance of ∼0.75 kΩ µm and Schottky barrier height of ∼3.3 meV, enabling nearly a 20-fold increase of carrier mobility in PdSe2 transistors compared to that of traditional Ti/Au contacts. This finding opens new possibilities in the development of better electrical contacts for practical applications of 2D materials.

Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe2 Transistors.

Atmospheric and long-term aging effects on electrical properties of WSe2 transistors with various thicknesses are examined. Although countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe2 pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H2O on pristine WSe2, which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O2 and H2O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H2O-pristine WSe2. The residual/permanent aging component is attributed to electron trapping molecular O2 and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.

3D Imaging and Manipulation of Subsurface Selenium Vacancies in PdSe_{2}.

Two-dimensional materials such as layered transition-metal dichalcogenides (TMDs) are ideal platforms for studying defect behaviors, an essential step towards defect engineering for novel material functions. Here, we image the 3D lattice locations of selenium-vacancy V_{Se} defects and manipulate them using a scanning tunneling microscope (STM) near the surface of PdSe_{2}, a recently discovered pentagonal layered TMD. The V_{Se} show a characterisitc charging ring in a spatially resolved conductance map, based on which we can determine its subsurface lattice location precisely. Using the STM tip, not only can we reversibly switch the defect states between charge neutral and charge negative, but also trigger migrations of V_{Se} defects. This allows a demonstration of direct "writing" and "erasing" of atomic defects and tracing the diffusion pathways. First-principles calculations reveal a small diffusion barrier of V_{Se} in PdSe_{2}, which is much lower than S vacancy in MoS_{2} or an O vacancy in TiO_{2}. This finding opens an opportunity of defect engineering in PdSe_{2} for such as controlled phase transformations and resistive-switching memory device application.

Ion Migration Studies in Exfoliated 2D Molybdenum Oxide via Ionic Liquid Gating for Neuromorphic Device Applications.

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. We present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO3) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO3 conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight-secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO3 devices as neuromorphic computing elements due to their synaptic plasticity.

Photocarrier Transfer across Monolayer MoS2-MoSe2 Lateral Heterojunctions.

In-plane heterojuctions formed from two monolayer semiconductors represent the finest control of electrons in condensed matter and have attracted significant interest. Various device studies have shown the effectiveness of such structures to control electronic processes, illustrating their potentials for electronic and optoelectronic applications. However, information about the physical mechanisms of charge carrier transfer across the junctions is still rare, mainly due to the lack of adequate experimental techniques. Here we show that transient absorption measurements with high spatial and temporal resolution can be used to directly monitor such transfer processes. We studied MoS2-MoSe2 in-plane heterostructures fabricated by chemical vapor deposition and lithographic patterning followed by laser-generated vapor sulfurization. Transient absorption measurements in reflection geometry revealed evidence of exciton transfer from MoS2 to MoSe2. By comparing the experimental data with a simulation, we extracted an exciton transfer velocity of 104 m s-1. These results provide valuable information for understanding and controlling in-plane carrier transfer in two-dimensional lateral heterostructures for their electronic and optoelectronic applications.

PdSe2: Pentagonal Two-Dimensional Layers with High Air Stability for Electronics.

Most studied two-dimensional (2D) materials exhibit isotropic behavior due to high lattice symmetry; however, lower-symmetry 2D materials such as phosphorene and other elemental 2D materials exhibit very interesting anisotropic properties. In this work, we report the atomic structure, electronic properties, and vibrational modes of few-layered PdSe2 exfoliated from bulk crystals, a pentagonal 2D layered noble transition metal dichalcogenide with a puckered morphology that is air-stable. Micro-absorption optical spectroscopy and first-principles calculations reveal a wide band gap variation in this material from 0 (bulk) to 1.3 eV (monolayer). The Raman-active vibrational modes of PdSe2 were identified using polarized Raman spectroscopy, and a strong interlayer interaction was revealed from large, thickness-dependent Raman peak shifts, agreeing with first-principles Raman simulations. Field-effect transistors made from the few-layer PdSe2 display tunable ambipolar charge carrier conduction with a high electron field-effect mobility of ∼158 cm2 V-1 s-1, indicating the promise of this anisotropic, air-stable, pentagonal 2D material for 2D electronics.

ANATOMICAL SITES OF COLORECTAL CANCER IN A SEMI-URBAN NIGERIAN HOSPITAL: IS THERE A TRUE RIGHTWARD SHIFT.

BACKGROUND: Recent report on colorectal tumours in Nigeria and Africa sub-region from big urban cities have shown that the incidence of colorectal cancer is rising and with a proportionate right-ward shift. OBJECTIVE: To assess the sub-site distribution and surgical treatmentpatterns of colorectal cancer in a semi-urban tertiary Nigerian hospital. DESIGN: A retrospective descriptive study. SETTING: Tertiary health institution in a semi-urban (rural) community. SUBJECTS: All consecutive in-patients, admitted and managed for acute intestinal obstruction due to colorectal cancer. MAIN OUTCOME: The right colon was the site of predilection (60.6%) while the left colon was only (6.06%). Young adults were mostly affected. RESULT: A total of thirty three (33) cases of colorectal cancer (CRC) were seen during the study period. There were 20 (60.6%) males and 13 (39.4%) females. The male:female ratio was 1.5:1. The age range was between 22-87 years (mean 57.09 years). The peak age of occurrence was the 6th decade. All patients'were symptomatic at presentation. The caccum (36.365) was the predominant site affected; followed by the rectum (24.24%) and the hepatic flexure (21.21%). Of the colonic tumours, 60.6% were on the right colon while only 6.06% were on the left colon. Tumours of the descending and sigmoid colon were conspicuously absent. Fifteen patients (45.5%) had curative resection and 33.3% had palliative surgery. Twenty one point two percent mortality was recorded, while 21.2% refused surgery and opted for alternative care. CONCLUSION: Colorectal cancers are seen among rural dwellers in Nigeria. Presentations are often late. The right colon is the dominant site affected; this calls for a change in strategy for formulating a preventive policy for the country.

Laparoscopic cholecystectomy in Ile-Ife, Nigeria.

BACKGROUND: Laparoscopic cholecystectomy is not yet routinely performed in most public tertiary hospitals in Nigeria. OBJECTIVES: To assess the feasibility and early outcoe of laparoscopic cholecystectoy in Ile-ife, Nigeria. METHOD: Consecutive patients who were selected for the procedure from June 2009 through December 2010 at the Ife State Hospital of the ObafemiAwolowo University Teaching Hospitals Complex, Ile-Ife were prospectively studied. RESULTS: Eighteen female (75%) and six male patients had laparoscopic cholecystectomy within the study period. They were aged 19-83 years with a mean age of 33.4 years. Six (25%) patients had acute calculous cholecystitis while the rest presented with chronic calculous cholecystitis. Their Body Mass Indices ranged from 21 to 32kg/m2. Operation time ranged from 65 to 105 minutes. One procedure (4%) was converted to open due to bleeding from the gallbladder bed. Another patient who had a minor common bile duct injury presented with generalized dull abdominal pain on the sixth postoperative day. She had open repair of the injury over a T-tube. Three patients (12.5%) were discharged on the first postoperative day while 15 (62.5%) others were discharged on the second postoperative day. No mortality was recorded. CONCLUSION: Laparoscopic cholecystectomy is feasible in our setting with outcome comparable to previous pioneering experiences in other African countries centre in spite of resource limitations.

Complete recovery of renal allograft function after sixty days of delay following living related transplantation.

Delayed graft function (DGF), a term employed when a newly transplanted organ does not function efficiently is commonly observed following cadaveric renal transplantation but is very rare after living related transplants. We present a 31-year-old female recipient of a related donor kidney (mother) who had DGF following trans-plantation due to acute tubular necrosis, probably caused by partial allograft arterial thrombosis, which recovered function after 60 days. Appropriate use of allograft biopsy should be encouraged even in resource-limited settings lest the allograft be assumed to have failed irreversibly.

Pattern and validity of clinical diagnosis of upper gastrointestinal diseases in south-west Nigeria.

BACKGROUND: Diagnosis of upper gastrointestinal (UGI) diseases is often made on clinical grounds alone in Nigeria due to lack of endoscopic facilities. The validity of using such diagnosis is presently unknown. OBJECTIVE: The study aimed to determine: age and sex distribution of patients presenting for UGI endoscopy; pattern of clinical and endoscopic diagnoses in patients with UGI diseases; and, the validity of clinic-based diagnosis. METHODS: Medical records of patients presenting at Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Nigeria for UGI endoscopy between September 1999 and August 2003 were reviewed. Data was analysed for sensitivity, specificity, positive predictive value, and negative predictive value of clinical diagnosis using endoscopic diagnosis as "gold" standard. RESULTS: Males constituted 53.4% of subjects and mean age was 45 years (+/- 1.69 SD). Peptic ulcer disease (PUD) constituted 67.6% of referral diagnosis but 33.9% of endoscopic diagnosis. PUD had the highest sensitivity value (0.72) while gastritis had the least (0.04). Specificity ranged from 0.40 for PUD to 1.00 for corrosive oesophagitis. Positive predictive value ranged from 0.29 (oesophageal cancer) to 0.67 (corrosive oesophagitis) and negative predictive value ranged from 0.66 for gastritis to 0.99 for corrosive oesophagitis. CONCLUSION: The validity of clinical diagnosis in UGI conditions varied widely, and in general, there is poor agreement between clinical and endoscopic diagnoses.

Pancreatic pseudocyst in Ile-Ife revisited: 5 more cases.

BACKGROUND: Pancreatic pseudocyst is a major health problem in the developed countries and its incidence is noted to be getting higher among the Caucasians as a result of better diagnostic techniques. This retrospective study was done to review the cases of pancreatic pseudocyst seen between 1991 and 1999 at the Obafemi Awolowo University Teaching Hospitals, Ile-Ife, Nigeria. The 5 cases seen over this period of study were used to illustrate its rarity. METHOD: The case notes of the patients managed for pancreatic pseudocyst during the period under review were studied and analysed to evaluate the different modes of presentation, investigations done and the modes of treatment. RESULTS: The age range of the patients varied between 23 and 70 years with three of them being male while two were female. The most common presenting symptom and sign were abdominal pain and abdominal mass. Abdominal ultrasonography, chest X-ray, upper gastrointestinal endoscopy and laboratory blood tests were the investigative procedures carried out. The treatment modalities were either conservative or surgical (operative). One patient had spontaneous resolution of the pseudocyst, 2 had external drainage while the remaining 2 had cystogastrostomy. Two of the patients responded well to treatment and were being followed up in the Surgical Out Patient Clinic, one died within 24 hours of surgery while the other developed diabetes mellitus 3 months post surgery and was referred to the physicians for follow-up. CONCLUSION: Though pancreatic pseudocyst remains uncommon in Nigeria, ultrasonography can play an essential role in assisting diagnosis while successful management of this condition is still possible in the absence of current endoscopic techniques of drainage.

Attitudes toward kidney donation.

The Renal Unit of Obafemi Awolowo University Teaching Hospital Ile-Ife in Southwest Nigeria intends commencing a kidney transplantation program. This cross-sectional study aimed at examining the willingness of Nigerians to be living-related kidney donors. Three hundred and sixteen Nigerians (96 first-degree relatives of end-stage renal disease patients, 69 rural dwellers and 151 health workers) were interviewed regarding their willingness to donate kidneys using an interview schedule designed to elicit socio-demographic information, knowledge about kidney transplantation and attitude toward kidney donation. Sixty-two percent of health workers, 52.1% of the patients' relatives and 27.1% of rural dwellers expressed willingness to donate. Higher proportions of health workers and patients' relatives--compared with the rural dwellers--were willing to donate a kidney to their children, full-siblings and parents (P<0.05). The level of awareness about kidney transplantation was highest among health workers and least among rural dwellers (P<0.001). Altruism was the primary motivation for those willing to donate a kidney. The most important reason for refusal to donate was fear of adverse health consequences. Among the rural dwellers, never-married persons were more willing than the married to donate (P<0.05). Programs aimed at increasing awareness about the safety of kidney donation, reducing adverse beliefs about kidney donation, and encouraging altruistic tendencies will increase the availability of kidney donors.

Pattern and presentation of large bowel neoplasms in Nigerians.

It is generally believed that colonic neoplasms are uncommon amongst Blacks. One hundred and twenty seven cases of large bowel neoplasms were treated at Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife between 1981 and 1990. There were 84 males and 43 females. The mean age was 53 years. Duration of illness prior to presentation at the hospital varied between two weeks to three years with a mean time lag of 15 months. Intestinal obstruction was found in 83 patients and these also presented with anaemia. However massive rectal bleeding was noticed only in 38 cases. Most of the lesions were rectosigmoid. Some of these patients accounting for 87 cases (69 pc) refused permanent colostomy due to social embarrassment. Histopathologic appearance of the tumours were mostly adenocarcinoma in 97 cases. Thirty five patients died within the first year of their presentation and management. Twenty eight patients are still being followed up while other patients have been lost to follow up. This study shows that a sizeable number of patients suffer from colonic neoplasm in our community.

Typhoid and paratyphoid fever: a retrospective study.

Sixty-eight proved cases of typhoid and paratyphoid fever were reviewed in a retrospective study covering 5 years (1986-1990). Patients within the age range of 10 to 39 years constituted 82.3 per cent of cases and there was equal incidence in both sexes. The mean duration of illness before presentation was 9.67 days. The major clinical features were fever (97%), abdominal tenderness (-9.4%), headache and abdominal pain (70.58%) each). Intestinal perforation was the commonest complication (27.9%) with a male preponderance (M:F-3:1). Perforation occurred after the first week of illness in 73.7 per cent of cases. Fourteen out of the nineteen patients who perforated were not on therapy at the time of perforation and they constituted 80 per cent of those cases of mortality in which perforation played a role. Surgical management of perforation gave better results than conservative management (mortality rates of 16.7% and 40% respectively). Salmonella was sensitive to Chloramphenicol in all the cases where the organism was grown. There were 10 recorded deaths (14.9%) of whom 60 per cent (i. e. 6 patients) presented after two weeks of illness.

Management of rectal prolapse in Ile-Ife, Nigeria.

Complete rectal prolapse is uncommon in adults. Out of 129,525 patients treated at our teaching hospital over a five-year period, only 29 patients were managed with complete rectal prolapse. The total mean-age was 52 years with an approximate 1:2 male-female ratio. Two of the patients had their prolapse for 16 years before presentation. Major clinical features included constipation, diarrhoea, soiling and rectal bleeding. 51.72% of the cases had partial to complete incontinence of faeces. 22 patients were treated with the simple technique posterior fixation of both rectum and sigmoid colon. Follow-up was from 6 months to 4 years, mortality was 3.44%. There had been no recurrences of the complete rectal prolapse to date.