Robust estimation of charge carrier diffusivity using transient photoluminescence microscopy.

Transient microscopy has emerged as a powerful tool for imaging the diffusion of excitons and free charge carriers in optoelectronic materials. In many excitonic materials, extraction of diffusion coefficients can be simplified because of the linear relationship between signal intensity and local excited state population. However, in materials where transport is dominated by free charge carriers, extracting diffusivities accurately from multidimensional data is complicated by the nonlinear dependence of the measured signal on the local charge carrier density. To obtain accurate estimates of charge carrier diffusivity from transient microscopy data, statistically robust fitting algorithms coupled to efficient 3D numerical solvers that faithfully relate local carrier dynamics to raw experimental measurables are sometimes needed. Here, we provide a detailed numerical framework for modeling the spatiotemporal dynamics of free charge carriers in bulk semiconductors with significant solving speed reduction and for simulating the corresponding transient photoluminescence microscopy data. To demonstrate the utility of this approach, we apply a fitting algorithm using a Markov chain Monte Carlo sampler to experimental data on bulk CdS and methylammonium lead bromide (MAPbBr3) crystals. Parameter analyses reveal that transient photoluminescence microscopy can be used to obtain robust estimates of charge carrier diffusivities in optoelectronic materials of interest, but that other experimental approaches should be used for obtaining carrier recombination constants. Additionally, simplifications can be made to the fitting model depending on the experimental conditions and material systems studied. Our open-source simulation code and fitting algorithm are made freely available to the scientific community.

Colloidal nano-MOFs nucleate and stabilize ultra-small quantum dots of lead bromide perovskites.

The development of synthetic routes to access stable, ultra-small (i.e. <5 nm) lead halide perovskite (LHP) quantum dots (QDs) is of fundamental and technological interest. The considerable challenges include the high solubility of the ionic LHPs in polar solvents and aggregation to form larger particles. Here, we demonstrate a simple and effective host-guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites. Cr3O(OH)(H2O)2(terephthalate)3 (Cr-MIL-101), made of large mesopore-sized pseudo-spherical cages, allows fast and efficient diffusion of perovskite precursors within its pores, and promotes the formation of stable, ∼3 nm-wide lead bromide perovskite QDs. CsPbBr3, MAPbBr3 (MA+ = methylammonium), and (FA)PbBr3 (FA+ = formamidinium) QDs exhibit significantly blue-shifted emission maxima at 440 nm, 446 nm, and 450 nm, respectively, as expected for strongly confined perovskite QDs. Optical characterization and composite modelling confirm that the APbBr3 (A = Cs, MA, FA) QDs owe their stability within the MIL-101 nanocrystals to both short- and long-range interfacial interactions with the MOF pore walls.

Tunable exciton binding energy in 2D hybrid layered perovskites through donor-acceptor interactions within the organic layer.

The strength of electrostatic interactions within semiconductors strongly affects their performance in optoelectronic devices. An important target is the tuning of a material's exciton binding energy-the energy binding an electron-hole pair through the electrostatic Coulomb force-independent of its electronic band gap. Here, we report on the doping of a family of two-dimensional hybrid perovskites, in which inorganic lead halide sheets alternate with naphthalene-based organic layers, with tetrachloro-1,2-benzoquinone (TCBQ). For four out of seven n = 1 perovskites, the incorporation of the electron-accepting TCBQ dopant into the organic sublattice containing the electron-donating naphthalene species enabled the tuning of the materials' 1s exciton binding energy. The naphthalene-TCBQ electron donor-acceptor interactions increased the electrostatic screening of the exciton, in turn lowering its binding energy relative to the undoped perovskite-by almost 50% in one system. Structural and optical characterization showed that the inorganic lattice is not significantly perturbed even though the layer-to-layer spacing increases upon molecular dopant incorporation.

Multi-cation perovskites prevent carrier reflection from grain surfaces.

The composition of perovskite has been optimized combinatorially such that it often contains six components (AxByC1-x-yPbXzY3-z) in state-of-art perovskite solar cells. Questions remain regarding the precise role of each component, and the lack of a mechanistic explanation limits the practical exploration of the large and growing chemical space. Here, aided by transient photoluminescence microscopy, we find that, in perovskite single crystals, carrier diffusivity is in fact independent of composition. In polycrystalline thin films, the different compositions play a crucial role in carrier diffusion. We report that methylammonium (MA)-based films show a high carrier diffusivity of 0.047 cm2 s-1, while MA-free mixed caesium-formamidinium (CsFA) films exhibit an order of magnitude lower diffusivity. Elemental composition studies show that CsFA grains display a graded composition. This curtails electron diffusion in these films, as seen in both vertical carrier transport and surface potential studies. Incorporation of MA leads to a uniform grain core-to-edge composition, giving rise to a diffusivity of 0.034 cm2 s-1 in CsMAFA films. A model that invokes competing crystallization processes allows us to account for this finding, and suggests further strategies to achieve homogeneous crystallization for the benefit of perovskite optoelectronics.

Direct Time-Domain View of Auger Recombination in a Semiconductor.

The radiationless recombination of electron-hole pairs in semiconductors is detrimental to optoelectronic technologies. A prominent mechanism is Auger recombination, in which nonradiative recombination occurs efficiently by transferring the released energy-momentum to a third charge carrier. Here we use femtosecond photoemission to directly detect Auger electrons as they scatter into energy and momentum spaces from Auger recombination in a model semiconductor, GaSb. The Auger rate is modulated by a coherent phonon mode at 2 THz, confirming phonon participation in momentum conservation. The commonly assumed Auger rate constant is found not to be a constant, but rather decreases by 4 orders of magnitude as hot electrons cool down by ∼90 meV. These findings provide quantitative guidance in understanding Auger recombination and in designing materials for efficient optoelectronics.

Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene.

The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1 may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 1(TT) state. Here, we probe the 1(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive 1(TT) state, which decays to 2×T1 with a time constant of 270 fs. However, the decay of S1 and the formation of 1(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S1→1(TT) rate process responsible for the 180 fs timescale, S1 may couple coherently to a vibronically excited 1(TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.

Screening in crystalline liquids protects energetic carriers in hybrid perovskites.

Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3 We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3 The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

Direct Observation of Entropy-Driven Electron-Hole Pair Separation at an Organic Semiconductor Interface.

How an electron-hole pair escapes the Coulomb potential at a donor-acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast time scales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor-vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination.

Charge Transfer Excitons at van der Waals Interfaces.

The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

Impact of implementing an electronic health record on surgical resident work flow, duty hours, and operative experience.

Our objective was to assess the effect of implementing an electronic health record (EHR) on surgical resident work flow, duty hours, and operative experience at a large teaching hospital. In May 2012, an EHR was put into effect at our institution replacing paper documentation and orders. Resident time to complete patient documentation, average duty hours, and operative experience before EHR and afterward (at 1, 4, 6, 8, and 24 weeks) were surveyed. We obtained 100 per cent response rate from 15 surgical residents at all time intervals. The average time spent documenting before EHR was 9 ± 2 minutes per patient document and at Weeks 1, 4, 6, 8, and 24 after EHR implementation was 22 ± 10, 15 ± 7, 15 ± 7, 14 ± 8, and 12 ± 4 minutes, respectively. Repeated measures analysis of variance demonstrated a difference among the means (P < 0.0001). Discharge summary and operative note remained significantly longer to complete at Week 24 compared with paper documentation (P < 0.05). Average resident work hours and operative cases per week before EHR were 77 ± 5 hours and 12 ± 5 cases, respectively, which were similar at all time points after EHR implementation (P > 0.05). At 24 weeks after EHR, 74 per cent of residents felt their risk of performing a medical error using electronic documentation and order entry was higher compared with paper charting and orders. Transition to EHR led to a significant doubling in resident time spent performing documentation for each patient. It improved over 6 months after implementation but never reached the pre-EHR baseline for operative notes and discharge summaries. Average resident work hours and case logs remained similar during this transition.

Arterial vs venous blood gas differences during hemorrhagic shock.

AIM: To characterize differences of arterial (ABG) and venous (VBG) blood gas analysis in a rabbit model of hemorrhagic shock. METHODS: Following baseline arterial and venous blood gas analysis, fifty anesthetized, ventilated New Zealand white rabbits were hemorrhaged to and maintained at a mean arterial pressure of 40 mmHg until a state of shock was obtained, as defined by arterial pH ≤ 7.2 and base deficit ≤ -15 mmol/L. Simultaneous ABG and VBG were obtained at 3 minute intervals. Comparisons of pH, base deficit, pCO2, and arteriovenous (a-v) differences were then made between ABG and VBG at baseline and shock states. Statistical analysis was applied where appropriate with a significance of P < 0.05. RESULTS: All 50 animals were hemorrhaged to shock status and euthanized; no unexpected loss occurred. Significant differences were noted between baseline and shock states in blood gases for the following parameters: pH was significantly decreased in both arterial (7.39 ± 0.12 to 7.14 ± 0.18) and venous blood gases (7.35 ± 0.15 to 6.98 ± 0.26, P < 0.05), base deficit was significantly increased for arterial (-0.9 ± 3.9 mEq/L vs -17.8 ± 2.2 mEq/L) and venous blood gasses (-0.8 ± 3.8 mEq/L vs -15.3 ± 4.1 mEq/L, P < 0.05). pCO2 trends (baseline to shock) demonstrated a decrease in arterial blood (40.0 ± 9.1 mmHg vs 28.9 ± 7.1 mmHg) but an increase in venous blood (46.0 ± 10.1 mmHg vs 62.8 ± 15.3 mmHg), although these trends were non-significant. For calculated arteriovenous differences between baseline and shock states, only the pCO2 difference was shown to be significant during shock. CONCLUSION: In this rabbit model, significant differences exist in blood gas measurements for arterial and venous blood after hemorrhagic shock. A widened pCO2 a-v difference during hemorrhage, reflective of poor tissue oxygenation, may be a better indicator of impending shock.

Outcomes of pancreatic debridement in acute pancreatitis: analysis of the nationwide inpatient sample from 1998 to 2010.

BACKGROUND: The objective of this study was to perform a national review of patients with acute pancreatitis (AP) who undergo pancreatic debridement (PD) to evaluate for risk factors of in-hospital mortality. METHODS: The Nationwide Inpatient Sample was used to identify patients with AP who underwent PD between 1998 and 2010. Risk factors for in-hospital mortality were assessed with multivariate logistic regression. RESULTS: From 1998 to 2010, there were 585,978 nonelective admissions with AP, of which 1,783 (.3%) underwent PD. From 1998 to 2010, the incidence of PD decreased from .44% to .25% (P < .01) and PD in-hospital mortality decreased from 29.0% to 15% (P < .05). Of patients undergoing PD, independent factors associated with increased odds of mortality were increased age (odds ratio [OR] 1.04, confidence interval [CI] 1.03 to 1.05; P < .01), sepsis with organ failure (OR 1.76, CI 1.24 to 2.51; P < .01), peptic ulcer disease (OR 1.83, CI 1.02 to 3.30; P < .05), liver disease (OR 2.27, CI 1.36 to 3.78; P < .01), and renal insufficiency (OR 1.78, CI 1.14 to 2.78; P < .05). CONCLUSIONS: The incidence and operative mortality of PD have decreased significantly over the last decade in the United States with higher odds of dying in patients who are older, with chronic liver, renal, or ulcer disease, and higher rates of sepsis with organ failure.

Impact of the establishment of a specialty hernia referral center.

BACKGROUND: Creating a surgical specialty referral center requires a strong interest, expertise, and a market demand in that particular field, as well as some form of promotion. In 2004, we established a tertiary hernia referral center. Our goal in this study was to examine its impact on institutional volume and economics. MATERIALS AND METHODS: The database of all hernia repairs (2004-2011) was reviewed comparing hernia repair type and volume and center financial performance. The ventral hernia repair (VHR) patient subset was further analyzed with particular attention paid to previous repairs, comorbidities, referral patterns, and the concomitant involvement of plastic surgery. RESULTS: From 2004 to 2011, 4927 hernia repairs were performed: 39.3% inguinal, 35.5% ventral or incisional, 16.2% umbilical, 5.8% diaphragmatic, 1.6% femoral, and 1.5% other. Annual billing increased yearly from 7% to 85% and averaged 37% per year. Comparing 2004 with 2011, procedural volume increased 234%, and billing increased 713%. During that period, there was a 2.5-fold increase in open VHRs, and plastic surgeon involvement increased almost 8-fold, (P = .004). In 2005, 51 VHR patients had a previous repair, 27.0% with mesh, versus 114 previous VHR in 2011, 58.3% with mesh (P < .0001). For VHR, in-state referrals from 2004 to 2011 increased 340% while out-of-state referrals jumped 580%. In 2011, 21% of all patients had more than 4 comorbidities, significantly increased from 2004 (P = .02). CONCLUSION: The establishment of a tertiary, regional referral center for hernia repair has led to a substantial increase in surgical volume, complexity, referral geography, and financial benefit to the institution.

The first nationwide evaluation of robotic general surgery: a regionalized, small but safe start.

BACKGROUND: The purpose of this study was to evaluate the outcomes of the most commonly performed robotic-assisted general surgery (RAGS) procedures in a nationwide database and compare them with their laparoscopic counterparts. METHODS: The Nationwide Inpatient Sample was queried from October 2008 to December 2010 for patients undergoing elective, abdominal RAGS procedures. The two most common, robotic-assisted fundoplication (RF) and gastroenterostomy without gastrectomy (RG), were individually compared with the laparoscopic counterparts (LF and LG, respectively). RESULTS: During the study, 297,335 patients underwent abdominal general surgery procedures, in which 1,809 (0.6 %) utilized robotic-assistance. From 2009 to 2010, the incidence of RAGS nearly doubled from 573 to 1128 cases. The top five RAGS procedures by frequency were LG, LF, laparoscopic lysis of adhesions, other anterior resection of rectum, and laparoscopic sigmoidectomy. Eight of the top ten RAGS were colorectal or foregut operations. RG was performed in 282 patients (0.9 %) and LG in 29,677 patients (99.1 %). When comparing RG with LG there was no difference in age, gender, race, Charlson comorbidity index (CCI), postoperative complications, or mortality; however, length of stay (LOS) was longer in RG (2.5 ± 2.4 vs. 2.2 ± 1.5 days; p < 0.0001). Total cost for RG was substantially higher ($60,837 ± 28,887 vs. $42,743 ± 23,366; p < 0.0001), and more often performed at teaching hospitals (87.2 vs. 50.9 %; p < 0.0001) in urban areas (100 vs. 93.0 %; p < 0.0001). RF was performed in 272 patients (3.5 %) and LF in 7,484 patients (96.5 %). RF patients were more often male compared with LF (38.2 vs. 32.3 %; p < 0.05); however, there was no difference in age, race, CCI, LOS, or postoperative complications. RF was more expensive than LF ($37,638 ± 21,134 vs. $32,947 ± 24,052; p < 0.0001), and more often performed at teaching hospitals (72.4 vs. 54.9 %; p < 0.0001) in urban areas (98.5 vs. 88.7 %; p < 0.0001). CONCLUSIONS: This nationwide study of RAGS exemplifies its low but increasing incidence across the country. RAGS is regionalized to urban teaching centers compared with conventional laparoscopic techniques. Despite similar postoperative outcomes, there is significantly increased cost associated with RAGS.

Postoperative quality of life after open transinguinal preperitoneal inguinal hernia repair using memory ring or three-dimensional devices.

A transinguinal preperitoneal (TIPP) approach has become a common technique for inguinal hernia repair. Our goal was to compare the impact of the two mesh designs for this operation: a flat mesh with a memory ring device (MRD) or a three-dimensional device (3DD) containing both onlay and preperitoneal mesh components. The prospective International Hernia Mesh Registry (2007 to 2012) was queried for MRD and 3DD inguinal hernia repairs. Outcomes and patient quality of life (QOL), using the Carolinas Comfort Scale (CCS), were examined at 1, 6, 12, and 24 months. Standard statistical methods were used, and multivariate logistic regression was performed using a forward stepwise selection method. TIPP was performed in 956 patients. Their average age 57.4 ± 15.3 years, 94.0 per cent were male, and mean body mass index was 25.7 ± 3.2 kg/m(2). MRD was used in 131 and 3DD in 825. Follow-up was 97, 82, 87, and 80 per cent at 1, 6, 12, and 24 months, respectively. Complications were not significantly different (P > 0.05). Recurrence was 0.8 per cent for MRD and 2.1 per cent for 3DD (P = 0.45). Comparing patient outcomes of MRD with 3DD at 1 month, 18.9 versus 11.5 per cent had symptoms of mesh sensation (P = 0.02); 28.7 versus 14.8 per cent had movement limitations (P < 0.01). MRD use was a significant independent predictor of movement limitation (odds ratio, 2.3; confidence interval, 1.4 to 3.7). No significant differences in CCS scores were seen at 6, 12, and 24 months. TIPP repair is safe and has a low recurrence rate. Early postoperative QOL is significantly improved with a 3DD mesh compared with MRD.

Does ventral hernia defect length, width, or area predict postoperative quality of life? Answers from a prospective, international study.

BACKGROUND: The goal of this study was to determine if ventral hernia defect length, width, or area predict postoperative pain and quality of life following ventral hernia repair (VHR). METHODS: The International Hernia Mesh Registry, a prospective database from 40 institutions worldwide, was queried for patients undergoing VHR from October 2007 to June 2012. Laparoscopic and open VHR were evaluated separately. Width and length were stratified into large, ≥10 cm and small, <10 cm, along with area as large, ≥100 cm(2) and small, <100 cm(2). RESULTS: In total, 865 International Hernia Mesh Registry patients underwent VHR. Large defect width, length, and area had no association with hernia recurrence or reoperation in both open and laparoscopic VHR. There was a significant increase in operating room time and length of stay for large compared with small width, length, and area for open and laparoscopic VHR patients (P < 0.05). Large area was associated with increased seroma and ileus in open and laparoscopic VHR (P < 0.05). There was greater pain and activity limitation at 1 mo for large versus small width and area whether repaired laparoscopically or open (P < 0.05). When comparing large to small length, there was no difference in pain for all follow-up time points when repaired laparoscopically, but there is significantly increased odds of pain and activity limitation at 1, 6, and 12 mo when repaired open (P < 0.05). CONCLUSIONS: Patients undergoing laparoscopic or open VHR with large defect widths and total area have a greater chance of pain and activity limitation at 1-mo follow-up, but not long term. Large defect lengths are associated with increased early and chronic discomfort in open VHR only.

Preliminary results of surgical and quality of life outcomes of Physiomesh in an international, prospective study.

Physiomesh is a novel, lightweight, large pore, polypropylene mesh designed to have flexibility that matches the compliance of the abdominal wall in an effort to improve patient quality of life (QOL). The International Hernia Mesh Registry was queried for ventral hernia repair (VHR) and inguinal hernia repair (IHR) with Physiomesh. Demographics, operative and postoperative details, and the Carolinas Comfort Scale (CCS) as a measure of QOL were recorded. Physiomesh was used in 100 patients, 29 IHR and 71 VHR. Their average age was 56.8 +/- 13.7, and BMI was 34.0 +/- 21.0 kg/m2. For IHR, preoperative pain (CCS > or = 2) was present in 41%, but decreased at 1, 6, and 12 months postoperatively to 25.9%, 0%, and 1.6%, while movement limitation decreased from 42.9% to 18.5%, 1.6%, and 3.1%. There were no complications or recurrences. The average VHR measured 66.4 cm2; 93% underwent a laparoscopic repair. Pain was present in 59.1% preoperatively but 21% at 12 months. Movement limitations reduced from 43.2% to 15.8% at 12 months. Mesh sensation was reported in only 10.5% at 1 year. There was 1 recurrence. Physiomesh is well tolerated by patients undergoing IHR and VHR. It is associated with a very favorable long-term QOL.

Surgical relief work in Haiti: a practical resident learning experience.

INTRODUCTION: A once-a-year, week-long surgical missionary trip to Haiti has become incorporated into our residency experience on a voluntary basis since 2007. The purpose of this article is to describe our experience with this mission effort during the last 4 years. METHODS: Since 2007, at least one PGY 3-5 surgical resident from our program has traveled to the Hôpital Sacré Coeur in Milot, Haiti for a voluntary, week-long surgical mission working with the local health care providers. Their personal and clinical experiences in Haiti, in the surgical clinics, and in the operating room, were recorded. RESULTS: Since 2007, 6 surgical residents and members of the surgical staff have traveled to Haiti for this surgical mission. During that time, a total of 247 patients were observed in the clinic and 184 surgical cases were performed. The case distribution covered a wide range of defined categories, including head and neck, breast, hernia, abdominal, biliary, stomach, small and large bowel, colorectal, skin and soft tissue, and urology. The personal aspect of this experience could not be quantitated but was profound. CONCLUSIONS: We feel that the surgical missionary trip to Haiti is an asset to our program. It provides humanitarian surgical care to patients in need, teaching and infrastructure support to the local health care providers, a clinical and operative experience to our residents, and an invaluable personal experience.