Surface Disorder Research Articles

Immunobiologicals and ocular surface disease.

Immunobiologicals have surfaced to become a new cornerstone of treatment for a wide spectrum of inflammatory disorders with an immune basis. The targets have ranged from autoimmune conditions to transplantation, and now more into atopic inflammatory disorders with primary targets of asthma and atopic dermatitis. The clinical information garnered from these studies have provided an initial snapshot on the potential adverse effects of the immunobiologicals on the ocular surface as well as providing a potential opening of their use in the treatment of various chronic ocular surface and intraocular inflammatory disorders that have previously been relegated to limited therapeutic options primarily to the broad anti-inflammatory use of glucocorticosteroids. The use of immunobiologicals provides a more directed therapeutic approach to many of the ocular inflammatory disorders, as we continue to appreciate more of their underlying immunopathophysiology of ocular surface disorder,s including ocular allergy and tear film dysfunction.

Vacancy driven surface disorder catalyzes anisotropic evaporation of ZnO (0001) polar surface

The evaporation and crystal growth rates of ZnO are highly anisotropic and are fastest on the Zn-terminated ZnO (0001) polar surface. Herein, we study this behavior by direct atomic-scale observations and simulations of the dynamic processes of the ZnO (0001) polar surface during evaporation. The evaporation of the (0001) polar surface is accelerated dramatically at around 300 °C with the spontaneous formation of a few nanometer-thick quasi-liquid layer. This structurally disordered and chemically Zn-deficient quasi-liquid is derived from the formation and inward diffusion of Zn vacancies that stabilize the (0001) polar surface. The quasi-liquid controls the dissociative evaporation of ZnO with establishing steady state reactions with Zn and O2 vapors and the underlying ZnO crystal; while the quasi-liquid catalyzes the disordering of ZnO lattice by injecting Zn vacancies, it facilitates the desorption of O2 molecules. This study reveals that the polarity-driven surface disorder is the key structural feature driving the fast anisotropic evaporation and crystal growth of ZnO nanostructures along the [0001] direction.

Proximity-induced superconductivity generated by thin films: Effects of Fermi surface mismatch and disorder in the superconductor

We investigate the effects of disorder characterising a superconducting thin film on the proximity-induced superconductivity generated by the film (in, e.g., a semiconductor) based on the exact numerical analysis of a three-dimensional microscopic model. To make the problem numerically tractable, we use a recursive Green's function method in combination with a patching approach that exploits the short-range nature of the interface Green's function in the presence of disorder. As a result of the Fermi surface mismatch between the superconductor (SC) and the semiconductor (SM) in combination with the confinement-induced quantization of the transverse SC modes, the proximity effect induced by a clean SC film is typically one to three orders of magnitude smaller that the corresponding quantity for a bulk SC and exhibits huge thickness-dependent variations. The presence of disorder has competing effects: on the one hand it enhances the proximity-induced superconductivity and suppresses its strong thickness dependence, on the other hand it generates proximity-induced effective disorder in the SM. The effect of proximity-induced disorder on the topological superconducting phase and the associated Majorana modes is studied nonperturbatively.

Evaluation of Tear Secretion and Tear Film Stability in Patients with Pterygium and Normal Individuals

Introduction: Abnormalities of any of the components of lacrimal function unit can lead to dysfunctional tear film and development of dry eye. Dry eye is a commonly encountered problem in general population.
 Materials and Methods: A hospital-based, case-control study was conducted at National Medical College and Teaching Hospital, Nepal from 1st August 2018 to 30th July 2019. A total of 279 participants: 93 cases of pterygium (Group A: case) and186 (Group B: control) normal individuals with no ocular surface disorder were enrolled. Objective analysis was done by Schirmer’s (SCH) without and with anesthesia and Tear Break up Time (TBUT). Dependent t test was used to analyze the data. P value <0.05 was considered statistically significant.
 Results: The mean age of the patients was 47.82 ±11.09 years (range 26-71) with male being 48 (51.6%) in Group A and in Group B, 101 (54.3%) were male with mean age being 42.16 ±13.13 years (range 32-75). The mean SCH without and with anesthesia and TBUT values were 16.94±5.61mm, 13.81±5.06mm, 10.14± 4.06 sec in Group A whereas in Group B, 20.39 ±6.08mm, 16.74 ±5.25mm and 13.05 ±4.32 sec respectively which was statistically significant (p < 0.05). Patients with pterygium had higher risk of having dry eye than normal individuals (Odd ratios: 2.69, 2.44 and 2.27 respectively).
 Conclusion: Pterygium is a common ocular surface disorder which could result in instability of tear film indices, dysfunctional tear film and thus dry eye.

Benzalkonium chloride, a common ophthalmic preservative, compromises rat corneal cold sensitive nerve activity

PurposeCorneal nerves comprise the densest sensory network in the body. Dysfunction of the corneal cold sensitive neurons (CSN) is implicated in ophthalmic disorders, including Dry Eye Disease, the most common ocular surface disorder. The preservative Benzalkonium chloride (BAK) and the mydriatic agent Phenylephrine hydrochloride (PHE) are considered to be inactive at the level of the CSNs. The purpose of this study is to test the impacts of continuous exposures to BAK or PHE at their clinically used concentrations on corneal nerve structure and function. MethodsIn vivo extracellular electrophysiology of the rat trigeminal ganglion was used to monitor CSN functional response to stimuli mimicking physiological states and stressors of the cornea. Corneal nerve structure was evaluated by immunostaining. ResultsAmong the tested stimuli, cold probe receptive field stimulation and hyperosmolar stress were the most sensitive methods of detecting activity changes. CSN activity was attenuated after 30 min exposure to either PHE or BAK. After an hour-long washout period, BAK-treated neurons failed to recover activity while PHE-treated neurons showed signs of functional recovery. Intraepithelial nerve density was reduced and nerve fragmentation was increased in BAK-treated corneas, while PHE exposure left corneal nerves structurally intact. ConclusionsOur study suggests that prolonged ocular instillations of BAK or PHE alter CSN activity through two different processes — irreversible neuronal damage in the case of BAK vs. reversible attenuation in the case of PHE.

A symmetry-based approach to the characterization of complex surface morphologies: Application in CuO and NiO nanostructures

In recent years, bottom-up nanofabrication techniques and especially solution-based ones have gained significant popularity due to their relatively facile implementation, low-cost and design flexibility that allows the development of a plethora of three-dimensional nanostructures and nanoarchitectures. Nonetheless, despite the apparent simplicity of the methods, the resulting nanostructures are often morphologically and hierarchically complex with no apparent way of appropriately describing their features in a quantifiable and methodological way. In this work, we propose a symmetry-based approach for the quantitative characterization of the morphology of nanostructured surfaces. The key idea is to characterize the morphology of nanostructures by means of the fundamental spatial symmetry metrics and the deviations from the fully symmetrical counterparts. The focus of the proposed methodology is on the translational and scaling symmetry, which are investigated through the mathematical tools of Fourier and (multi)fractal analysis, respectively. The calculated metrics characterizing these symmetries are the period and the fractal dimension of surfaces while the deviations are quantified by the disorder parameter ω d related to the FWHM of Fourier peak and by the asymmetry of the multifractal spectrum for the translational and scaling symmetry respectively. As an initial study case and in order to demonstrate its suitability, the developed symmetry-based methodology was applied to nickel oxide (NiO) and copper oxide (CuO) nanostructures hydrothermally-grown on silicon substrates. The results show that even in the cases where the surfaces display similar symmetry metrics (period, fractal dimension) the symmetry-based approach reveals subtle, but quantifiable differences in the deviations from full symmetries which are not easily identified by other approaches. • A symmetry-based method for nanometrological analysis of complex nanomorphologies is developed and implemented. • It detects and quantifies the spatial symmetries in surfaces and the deviations from these (surface disorder). • Fourier, correlation and (multi) fractal analysis are used. • The method is successfully applied in CuO and NiO nanostructures hydrothermally grown on silicon substrates. • It captures subtle differences in nanosurface translational and scaling disorder and evaluates material/process origins.

The road to diagnosis in VKC: a qualitative study of patients' and caregivers' lived experience

Vernal keratoconjunctivitis (VKC) is a rare allergic inflammatory ocular surface disorder (OSD) that primarily affects children and young adults. Diagnostic delays are common, and little is known of the patient experience in the time leading up to diagnosis.

Case Control Study to Evaluate Tear Film Function and Ocular Surface Disorder in Diabetic Patients

BACKGROUND
 Diabetic retinopathy and early development of cataracts are well-known ocular complications of long-standing diabetes. In recent years various researchers have reported ocular surface disorders, particularly dry eyes in diabetic patients which if left untreated may result in a variety of corneal complications. These ocular surface disorders can be clinically diagnosed by performing a detailed slit lamp examination, Schirmer's test and tear film break-up time. Our study intended at assessing tear film and ocular surface disorders in diabetic patients and study tear film function in them and compare it with non-diabetic patients.
 METHODS
 A comparative cross-sectional study was done on 100 diabetic and 100 non-diabetic patients > 30 years of age visiting OPD of health centre of tertiary care hospital of Central India. Clinical data and detailed clinical history were obtained from all subjects through direct patient interviews and reviewing of their medical records followed by a comprehensive ocular examination that included best-corrected vision, slit lamp examination, and Tear Film break up time (TBUT) & Schirmer test.
 RESULTS
 In our study, mean TBUT was found to be significantly reduced in diabetics (15.94 ± 3.8) as compared to non-diabetics (20.13 ± 6.6). No significant difference was found in the Schirmer test reading between diabetics (17.13 ± 3.20) and non-diabetics (19.20 ± 6.23). Diabetic male patients had significantly decreased mean TBUT & mean Schirmer test readings as compared to diabetic females. Mean Schirmer reading & Mean TBUT were lowest in a patients having > 20 years of DM and highest in patients having < 5 years of DM. No positive correlation was found between HbA1c (Glyosylated Haemoglobin) and diabetic retinopathy with TBUT/ Schirmer test readings.
 CONCLUSIONS
 Our study showed that the tear film stability was found to be affected in diabetic patients, as compared to non-diabetic subjects as evident by the lower level of TBUT in diabetic patients. Significantly reduced TBUT with no significant difference in Schirmer test reading in diabetic as compared to non-diabetic subjects might be due to a decrease in basal secretion with normal overall tear secretion. The absence of subjective symptoms of dryness in spite of decreased TBUT in the diabetic group might be due to a decrease in corneal sensitivity caused by diabetic peripheral neuropathy in diabetic patients. There was an increase in the prevalence of DES with an increasing duration of DM. Rheumatoid arthritis was found to be a risk factor for DES. Diabetic males were found to be at risk for developing dry eye syndrome. Hence it can be concluded from our study that every diabetic patient should undergo a comprehensive eye examination which should include various tests for dry eye assessing multiple tear parameters.

Atomistic origin of nano-silver paracrystalline structure: molecular dynamics and x-ray diffraction studies

Classical molecular dynamics (MD) and x-ray diffraction (XRD) have been used to establish the origin of the paracrystalline structure of silver nanoparticles at the atomic scale. Models based on the face-centred cubic structure have been computer generated and their atomic arrangements have been optimized by the MD with the embedded-atom model (EAM) potential and its modified version (MEAM). The simulation results are compared with the experimental XRD data in reciprocal and real spaces, i.e. the structure factor and the pair distribution function. The applied approach returns the structural models, defined by the Cartesian coordinates of the constituent atoms. It has been found that most of the structural features of Ag nanoparticles are better reproduced by the MEAM. The presence of vacancy defects in the structure of the Ag nanoparticles has been considered and the average concentration of vacancies is estimated to be 3 at.%. The average nearest-neighbour Ag–Ag distances and the coordination numbers are determined and compared with the values predicted for the bulk Ag, demonstrating a different degree of structural disorder on the surface and in the core, compared to the bulk crystalline counterpart. It has been shown that the paracrystalline structure of the Ag nanoparticles has origin in the surface disorder and the disorder generated by the presence of the vacancy defects. Both sources lead to network distortion that propagates proportionally to the square root of the interatomic distances.

Ubiquitous defect-induced density wave instability in monolayer graphene.

Quantum materials are notoriously sensitive to their environments, where small perturbations can tip a system toward one of several competing ground states. Graphene hosts a rich assortment of such competing phases, including a bond density wave instability (“Kekulé distortion”) that couples electrons at the K/K′ valleys and breaks the lattice symmetry. Here, we report observations of a ubiquitous Kekulé distortion across multiple graphene systems. We show that extremely dilute concentrations of surface atoms (less than three adsorbed atoms every 1000 graphene unit cells) can self-assemble and trigger the onset of a global Kekulé density wave phase. Combining complementary momentum-sensitive angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED) measurements, we confirm the presence of this density wave phase and observe the opening of an energy gap. Our results reveal an unexpected sensitivity of the graphene lattice to dilute surface disorder and show that adsorbed atoms offer an attractive route toward designing novel phases in two-dimensional materials.

Phase engineering activation of low-cost iron-containing sulfide minerals for advanced electrocatalysis

• A phase transformation strategy was designed for activating natural minerals. • These activated minerals could be used for advanced electrocatalysis. • The haycockite phase showed the best performance for acidic HER and alkaline OER. • Further optimization was performed by loading iridium clusters for alkaline OER. Sustainable energy conversion and storage provide feasible approaches towards green energy solutions and carbon neutralization. The high cost and complex fabrication process of advanced energy nanomaterials, however, has impeded the practical application of emerging sustainable technologies. The direct use of earth-abundant natural minerals which contain active elements for effective catalysis and energy storage should be a promising approach to achieve affordable sustainable energy supply and green fuel generations. Herein, as typical examples of activating natural minerals for electrocatalysis, two common minerals, pyrite and chalcopyrite, are activated via a one-step phase transformation strategy. Through a facile thermal reduction process, the minerals are completely transformed into active pyrrhotite (FeS) and haycockite (Cu 4 Fe 5 S 8 ) phases. The thermal reduction resulting phase transformation can lead to significant surface disordering and can contribute to the catalytic activity by offering favourable electronic structure for intermediates adsorption, abundant surficial active centres, and substantial surface redox pairs. The activated minerals are examined for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysis. The obtained haycockite phase delivers the best performance towards acidic HER and alkaline OER. Further phase optimization is performed via loading a low percentage of iridium nanoclusters on the haycockite phase deposited onto a carbon cloth substrate, through which an overpotential as low as 310 mV for achieving 10 mA cm −2 and a small Tafel slope of 55.6 mV dec −1 are recorded for alkaline OER. This work demonstrates the feasibility of the direct use of cost-effective natural resources for addressing the current energy-related issues and paves a way to reach affordable practical emerging sustainable technologies.

Ternary metal oxide substrates for superconducting circuits

Substrate material imperfections and surface losses are one of the major factors limiting superconducting quantum circuitry from reaching the scale and complexity required to build a practical quantum computer. One potential path towards higher coherence of superconducting quantum devices is to explore new substrate materials with a reduced density of imperfections due to inherently different surface chemistries. Here, we examine two ternary metal oxide materials, spinel (MgAl2O4) and lanthanum aluminate (LaAlO3), with a focus on surface and interface characterization and preparation. Devices fabricated on LaAlO3 have quality factors three times higher than those of earlier devices, which we attribute to a reduction in the interfacial disorder. MgAl2O4 is a new material in superconducting quantum devices, and even in the presence of significant surface disorder, it consistently outperforms LaAlO3. Our results highlight the importance of materials exploration, substrate preparation, and characterization for identifying materials suitable for high-performance superconducting quantum circuitry.

CO2 hydrogenation to olefins on supported iron catalysts: Effects of support properties on carbon-containing species and product distribution

Catalytic utilizing of CO2 into valuable olefins on Fe-based catalysts is a feasible way to mitigate the pressure of carbon neutrality. Herein, by investigating the effects of support (SiO2, Al2O3, CNT, and ZrO2) on the structure and catalytic performance of iron catalysts, we have found that ZrO2 supported catalysts are more beneficial to olefin selectivity and chain growth. The optimization catalytic performance of FeNa/ZrO2 displays C2-C7 olefin selectivity of 68.2% (O/P = 8.0) with a space time yields (111.4 g Kgcat-1h−1) at XCO2 = 32.7%, 320 ⁰C, 2.0 MPa, CO2/H2/Ar = 1:3:3, and 9000 mL g-1h−1. Characterizations show that the support regulates the composition of the iron oxide phase and the iron carbide phase and induces to form the highest content of Fe5C2 on the spent FeNa/ZrO2, which is probably caused by the carbon rich surface and lower disorder of carbon species. Besides, the higher intensity of CO2 and CO adsorption and lower intensity of H2 adsorption is confirmed on the surface of the FeNa/ZrO2 catalysts, leading to a higher ratio of C/H on the surface of the spent catalysts and consequently inhibiting the hydrogenation of olefins. Therefore, the O/P is 8 times as many as that on the FeNa/SiO2 and FeNa/Al2O3. Understanding the effect of support on CO2 hydrogenation to olefins over iron-based catalysts would be significant for precisely designing new catalysts.

Экспериментальная модель хронического воспаления тканей глазной поверхности, ассоциированного с медикаментозным лечением глаукомы

Purpose. To study experimental model of eye surface chronic inflammation, associated with glaucoma medication. Material and methods. 40 rabbits received glaucoma medication containing prostaglandin analogue (PGA), beta-blocker (BB) and preservative benzalkonium chloride (PGA + BB group) for 3 months. Rabbits of control group (n=12) received drops of normal saline for the same period of time. All the rabbits underwent regular ophthalmic examination including biomicroscopy, conjunctival hyperemia scoring, corneal examination involving fluorescein staining as well as Shirmer's test I. In 3 months tissues of conjunctiva, Tenon's capsule and cornea were histologically investigated by means of microscopy. Results. Median score of conjunctival hyperemia was significantly higher at PGA + BB group by the end of the study: 2.0 (1.0; 2.0) vs 0.0 (0.0; 0.5) points in control group. Corneal epithelium damage score according to Efron grading scales was 2.0 (1.0; 2.0) points in PGA+BB group, while corneas of control group rabbits didn't show signs of fluorescein staining. Rabbits under topical glaucoma medication had lower results of Shirmer's test I: 10.0 (9.5; 11.0) mm vs 14.0 (14.0; 15.0) mm at animals of control group by the end of experiment. Histological examination revealed thickened conjunctiva, thickened conjunctival epithelium with disorganization of its layers, lesions of monocyte infiltration, fibrosis and neoangiogenesis in conjunctiva and Tenon's capsule, as well as local detachment of upper layers of corneal epithelium in PGA + BB group. Conclusion. We received experimental model of eye surface disease, associated with glaucoma medication, as a result of daily 3-months administration of fixed combination of PGA and BB containing preservative in rabbit eyes. Eye surface disorder was characterized by morphological signs of chronic inflammation of conjunctiva and Tenon's capsule with clinical manifestations of dry eye disease. Key words: glaucoma medication, preservatives, experimental model, dr y eye disease, inflammation, conjunctiva

PAF-induced inflammatory and immuno-allergic ophthalmic diseases and their mitigation with PAF receptor antagonists: Cell and nuclear effects.

Ocular allergies are becoming more prevalent as more airborne pollutants, irritants and microbes pervade our environment. Inflammatory and allergic mediators released by dendritic and mast cells within the conjunctiva cause allergic conjunctivitis (AC), a prevalent ocular surface disorder that affects >40% of the world's human population on a seasonal or perennial basis. Even though histamine is a major culprit, platelet-activating factor (PAF) also contributes to AC, acting either directly or synergistically with histamine and other mediators. PAF receptor-meditated inflammatory reactions, via cell-membrane-bound and nuclear-membrane-bound and nuclear PAF receptors, are also implicated in the etiology of other eye diseases such as uveitis, diabetic retinopathy, corneal and choroidal neovascularization, and age-related macular degeneration which cause serious visual impairment and can lead to blindness. This review highlights the various deleterious elements implicated in the pathological aspects of ocular allergic reactions and inflammation and provides concepts and treatment options to mitigate these eye disorders with a special focus on PAF and PAF receptor antagonists.

Mesenchymal Stem Cells as Therapeutic Tools in Ocular Surface Disorder and Lacrimal Gland Regeneration

Mesenchymal Stem Cells as Therapeutic Tools in Ocular Surface Disorder and Lacrimal Gland Regeneration

Simultaneously Accelerating Carrier Transfer and Enhancing O2/CH4 Activation via Tailoring the Oxygen-Vacancy-Rich Surface Layer for Cocatalyst-Free Selective Photocatalytic CH4 Conversion.

Solar energy-driven direct CH4 conversion to liquid oxygenates provides a promising avenue toward green and sustainable CH4 industry, yet still confronts issues of low selectivity toward single oxygenate and use of noble-metal cocatalysts. Herein, for the first time, we report a defect-engineering strategy that rationally regulates the defective layer over TiO2 for selective aerobic photocatalytic CH4 conversion to HCHO without using noble-metal cocatalysts. (Photo)electrochemical and in situ EPR/Raman spectroscopic measurements reveal that an optimized oxygen-vacancy-rich surface disorder layer with a thickness of 1.37 nm can simultaneously promote the separation and migration of photogenerated charge carriers and enhance the activation of O2 and CH4, respectively, to •OH and •CH3 radicals, thereby synergistically boosting HCHO production in aerobic photocatalytic CH4 conversion. As a result, a HCHO production rate up to 3.16 mmol g-1 h-1 with 81.2% selectivity is achieved, outperforming those of the reported state-of-the-art photocatalytic systems. This work sheds light on the mechanism of O2-participated photocatalytic CH4 conversion on defective metal oxides and expands the application of defect engineering in designing low-cost and efficient photocatalysts.

Evaluating plasma assisted noninvasive surgery (PANIS) as a new method for pterygium removal; a clinical case series

Purpose: The present study aimed to evaluate the outcomes of a new technique for pterygium surgery using a plasma-based method. Methods: A clinical case series study was carried out in 6 patients. Six eyes with grade 2 or 3 of primary pterygium were operated. The pterygium head was detached from the corneal surface. Then, the free conjunctival edges were attached near the limbus using the plasma spots [generated by Plexr device (GMV, Rocca Priora, RM, Italy)]. Refractive Error (RE), Visual Acuity (VA), Intraocular Pressure (IOP), Contrast Sensitivity (CS), Corneal Fluorescein Staining (CFS), Tear Meniscus Height (TMH), High Order Aberrations (HOA), and Ocular Surface Disorder Index (OSDI) were measured before the operation, one week after the operation, as well asone, three, and six months post-surgery. Slit-lamp images were taken before surgery and six months after that. Results: No intraoperative and postoperative complications were observed. Mild degrees of pain, tearing, foreign body sensation, and irritation was reported during the first week after surgery. No significant change was found in IOP, CS, and TMH parameters during the 6-month follow-up. A slight improvement was measured in CFS and HOA after surgery. Compare to before surgery, RE, VA, and OSDI improved after pterygium removal. The follow-up examinations revealed pterygium recurrence in 2 eyes. Conclusion: Our results showed that plasma-assisted noninvasive surgery can provide a simple, fast, and effective techniquefor pterygium removal with no suture or glue. Although, considering the two observed recurrences in this study, we decided to modify the surgical method in the subsequent studies. Keywords: pterygium; treatment; plasma; noninvasive surgery; case series.

Fungal corneal ulcer in a 27 weeks primigravida with anemia and multi-vitamin deficiency

Fungal keratitis is an infection of the cornea which can lead to severe visual compromise if not diagnosed on time and managed appropriately. It may present with minimal symptoms and many signs. Likely to be overlooked by the patient, it results in a late presentation when substantial damage has already occurred. It is usually seen in patients with a severe immune compromise such as acquired immunodeficiency syndrome (AIDS), uncontrolled diabetes, transplant patients on immune-suppressants or long-term steroids, and in pre-existing ocular surface disorder. This is a case report of a 27-week primigravida who presented with extreme fatigue and a fungal corneal ulcer with no preceding history of trauma or any other immune compromised state.

Single-Crystal Alkali Antimonide Photocathodes: High Efficiency in the Ultrathin Limit.

The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this Letter, we demonstrate the synthesis of epitaxial thin films of Cs_{3}Sb on 3C-SiC (001) using molecular-beam epitaxy. Films as thin as 4nm have quantum efficiencies exceeding 2% at 532nm. We also find that epitaxial films have an order of magnitude larger quantum efficiency at 650nm than comparable polycrystalline films on Si. Additionally, these films permit angle-resolved photoemission spectroscopy measurements of the electronic structure, which are found to be in good agreement with theory. Epitaxial films open the door to dramatic brightness enhancements via increased efficiency near threshold, reduced surface disorder, and the possibility of engineering new photoemission functionality at the level of single atomic layers.