Electrocatalytic Reduction of N2 Using Metal-Doped Borophene.

Ammonia synthesis is an essential process in chemistry and industry. However, it is limited by the lack of efficient catalysts and high energy costs. Developing highly efficient systems for ammonia synthesis is an important and long-standing challenge. In this paper, a large class of metal atoms (including 3d/4d transition metals and main group metals) anchored onto borophene have been studied as single atom catalysts for ammonia synthesis. After comprehensive computational screening and systematic evaluation, four candidates stand out. We predict that Mo, Mn, Tc, and Cr@BM-ß12 will have superior performance for catalytic reduction of N2 to NH3 with low limiting potentials of -0.26, -0.32, -0.38, and -0.48 V, respectively. Furthermore, we studied the activity of the competitive HER on M@BM-ß12. The results implied that the two materials Mo@BM-ß12 and Mn@BM-ß12 showed HER suppression. These properties exceed most currently reported nitrogen reduction reaction electrocatalysts. Our results suggest the possibility of efficient electrochemical reduction of N2 to NH3 in a lower energy process.

Properties and Detailed Adsorption of CO2 by M2(dobpdc) with N,N-Dimethylethylenediamine Functionalization.

We have systematically investigated the CO2 adsorption performance and microscopic mechanism of N,N-dimethylethylenediamine (mm-2) appended M2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate; M = Mg, Sc-Zn) with density functional theory. These calculations show that the mm-2 has strong interactions with the open metal site of these structures via the first amine, and the mm-2 binding energies are generally between 123 and 172 kJ/mol. After the CO2 is attached, the ammonium carbamate molecule is created by insertion. The CO2 adsorption energies (31-81 kJ/mol) depend on the metal used (Mg; Sc-Zn). The microscopic mechanism of the CO2 adsorption process is presented at the atomic level, and the detailed potential energy surface and reaction path information are provided. The CO2 molecule and mm-2 grafted M2(dobpdc) are firstly combined via physical interactions, and then, the complex is converted into an N-coordinated zwitterion intermediate over a large energy barrier (1.02-1.51 eV). Finally, the structure is rearranged into a stable ammonium carbamate configuration through a small energy barrier (0.05-0.25 eV). We hope that this research will contribute to the understanding and production of real-world carbon capture materials.

Ammonia Synthesis Using Single-Atom Catalysts Based on Two-Dimensional Organometallic Metal Phthalocyanine Monolayers under Ambient Conditions.

We have identified three novel metal phthalocyanine (MPc, M = Mo, Re, and Tc) single-atom catalyst candidates with excellent predicted performance for the production of ammonia from electrocatalytic nitrogen reduction reaction (NRR) through a combination of high-throughput screening and first-principles calculations on a series of 3d, 4d, and 5d transition metals anchored onto extended Pc monolayer catalysts. Analysis of the energy band structures and projected density of states of N2-MPc revealed significant orbital hybridization and charge transfer between the adsorbed N2 and catalyst MPc, which accounts for the high catalytic activity. Among 30 MPc catalysts, MoPc and TcPc monolayers were found to be the most promising new NRR catalysts, as they exhibit excellent stability, low onset potential, and high selectivity. A comprehensive reaction pathway search found that the maximum free energy changes for the MoPc and TcPc monolayers are 0.33 and 0.54 eV, respectively. As a distinctive nature of this work, the hybrid reaction pathway was considered extensively and searched systematically. The onset potential of the hybrid pathway is found to be smaller than or comparable to that of the commonly known pure pathway. Thus, the hybrid path is highly competitive with low onset potential and high activity. The hybrid pathway is expected to have an important impact on future research on the mechanism of NRR, and it will open up a new way to explore the mechanism of the NRR reaction. We hope that our work will provide impetus to the creation of new catalysts for reduction of N2 to NH3. This work provides new insights into the rational design of NRR catalysts and explores novel reaction pathways under ambient or mild conditions.

Formation Mechanism of Ammonium Carbamate for CO2 Uptake in N,N'-Dimethylethylenediamine Grafted M2(dobpdc).

The adsorption properties and formation mechanism of ammonium carbamate for CO2 capture in N,N'-dimethylethylenediamine (mmen) grafted M2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate; M = Mg, Sc-Zn, except Ni) have been studied via density functional theory (DFT) calculations. We see that the mmen molecule is joined to the metal site via a M-N bond and has hydrogen bonding with neighboring mmen molecules. The binding energies of mmen range from 135.4 to 184.0 kJ/mol. CO2 is captured via insertion into the M-N bond of mmen-M2(dobpdc), forming ammonium carbamate. The CO2 binding energies (35.2 to 92.2 kJ/mol) vary with different metal centers. Furthermore, the Bader charge analysis shows that the CO2 molecules acquire 0.42 to 0.47 |e|. This charge is mainly contributed by the mmen, and a small additional amount is from the metal atom bonded with the CO2. The preferred reaction pathway is a two-step reaction. In the first step, the hydrogen bonded complex B changes into an N-coordinated intermediate D with high barriers (0.69 to 1.58 eV). The next step involves the translation and rotation of the chain in the intermediate D, resulting in the formation of the final O-coordinated product I with barriers of 0.22 to 0.61 eV. The higher barriers of CO2 reaction with mmen-M2(dobpdc) relative to attack the primary amine might be due to the larger steric hindrance of mmen. We hope this work will contribute to an improved understanding and development of future amine-grafted materials for efficient CO2 capture.

Disclosing the microscopic mechanism and adsorption properties of CO2 capture in N-isopropylethylenediamine appended M2(dobpdc) series.

The detailed picture of the microscopic mechanism for CO2 capture in N-isopropylethylenediamine (i-2) functionalized M2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate; M = Mg, Sc-Zn) has been determined for the first time via systematic computations with van der Waals (vdW) corrected density functional theory (DFT) methods. The results show that acting as a Lewis base, the i-2 molecule can strongly interact with the acidic open metal sites of M2(dobpdc) via its primary amine with binding energies of 132 to 178 kJ mol-1 for different metals. After exposure to gaseous CO2, CO2 is captured by inserting into the metal-N bond. The corresponding CO2 binding energies (43-69 kJ mol-1) vary depending on the metal centers. i-2-Sc2(dobpdc) and i-2-Mg2(dobpdc) with high CO2 binding energies have promising potential for CO2 capture. Moreover, the results demonstrate that the CO2 capture process involves two steps, consisting of simultaneous nucleophilic attack of the CO2 onto the metal-bound N atom with proton transfer. This results in the formation of a zwitterion intermediate (step1), and then rearrangement of the zwitterion intermediate into the final product ammonium carbamate (step2). The first step with relatively high barriers (0.99-1.49 eV) is rate-determining. The second step with low barriers (less than 0.50 eV) can easily occur and will promote the reaction. This work uncovers the complicated microscopic mechanism of CO2 capture with i-2 functionalized MOFs at the molecular level. This study provides fundamental understanding of the adsorption process and insights into the design and synthesis of highly efficient CO2 capture materials.

Elucidation of the Underlying Mechanism of CO2 Capture by Ethylenediamine-Functionalized M2(dobpdc) (M = Mg, Sc-Zn).

We, for the first time, systematically investigated the crystal structures, adsorption properties, and microscopic mechanism of CO2 capture with ethylenediamine (en)-appended isostructural M2(dobpdc) materials (M = Mg, Sc-Zn), using spin polarized density functional theory (DFT) calculations. The binding energies of en range from 142 to 210 kJ/mol. The weakest binding materials are en-Cr2(dobpdc) and en-Cu2(dobpdc). Two typical models, the pair model and the chain model, have been considered for CO2 adsorption. Generally, the chain model is more stable than the pair model. The CO2 adsorption energies of the chain model are in the range of 30-96 kJ/mol, with a strong metal dependence. Among these, the en-Sc2(dobpdc) and en-Cu2(dobpdc) have the highest and lowest CO2 adsorption energies, respectively. Moreover, the dynamic progress of CO2 adsorption has been unveiled via exploration of the full reaction pathway, including transition states and intermediates. First, the CO2 molecule interacts with en-MOFs to form a physisorbed complex with a shallow potential well. This is followed by overcoming a relatively large energy barrier to form a chemisorbed complex. Finally, ammonium carbamate is formed along the one-dimensional channels within the pore with a small energy barrier for configuration transformation. These results agree well with the experimental observations. Understanding the detailed microscopic mechanism of CO2 capture is quite crucial for improving our fundamental knowledge base and potential future applications. This work will improve our understanding of CO2 adsorption with amine functionalized MOFs. We expect our results to stimulate future experimental and theoretical research and advance the development of this field.

Intraoperative coagulopathy during cesarean section as an unsuspected initial presentation of COVID-19: a case report.

BACKGROUND: The world's understanding of COVID-19 continues to evolve as the scientific community discovers unique presentations of this disease. This case report depicts an unexpected intraoperative coagulopathy during a cesarean section in an otherwise asymptomatic patient who was later found to have COVID-19. This case suggests that there may be a higher risk for intrapartum bleeding in the pregnant, largely asymptomatic COVID-positive patient with more abnormal COVID laboratory values. CASE: The case patient displayed D-Dimer elevations beyond what is typically observed among this hospital's COVID-positive peripartum population and displayed significantly more oozing than expected intraoperatively, despite normal prothrombin time, international normalized ratio, fibrinogen, and platelets. CONCLUSION: There is little published evidence on the association between D-Dimer and coagulopathy among the pregnant population infected with SARS-CoV-2. This case report contributes to the growing body of evidence on the effects of COVID-19 in pregnancy. A clinical picture concerning for intraoperative coagulopathy may be associated with SARS-CoV-2 infection during cesarean sections, and abnormal COVID laboratory tests, particularly D-Dimer, may help identify the patients in which this presentation occurs.

Adsorption Properties and Microscopic Mechanism of CO2 Capture in 1,1-Dimethyl-1,2-ethylenediamine-Grafted Metal-Organic Frameworks.

The adsorption properties and microscopic mechanism of CO2 adsorption in 1,1-dimethyl-1,2-ethylenediamine (dmen) functionalized M2(dobpdc) (dobpdc4-=4,4'-dioxidobiphenyl-3,3'-dicarboxylate; M = Mg, Sc-Zn) have been completely unveiled for the first time via comprehensive investigations based on first-principles density functional theory (DFT) calculations. The results show that for the primary-primary amine, dmen prefers to interact with the open metal site of M2(dobpdc) via the end with smaller steric hindrance. The binding energies of dmen with MOFs are in the range of 104-174 kJ/mol. In presence of CO2, it fully inserts into the metal-N bond, forming ammonium carbamate. The CO2 binding energies vary from 53 to 89 kJ/mol, showing strong metal dependence. Among the 11 metals, dmen-Sc2(dobpdc) and dmen-Mg2(dobpdc) have the highest CO2 binding energies of 89 and 84 kJ/mol, respectively, and may have large CO2 adsorption capacity for practical applications. More importantly, the microscopic CO2 capture process of dmen-M2(dobpdc) is revealed at the atomic level. The whole reaction process includes two steps, that is, formation of zwitterion intermediate (step 1) and rearrangement of the zwitterion intermediate (step 2). The first step in which nucleophilic addition between CO2 and the metal-bound amine and proton transfer from the metal-bound amine to free amine simultaneously occur is a rate-determining step, with higher energy barriers (0.99-1.35 eV). The second step with much lower barriers (maximum of 0.16 eV) is extremely easy, which can promote the whole CO2 uptake process in dmen-M2(dobpdc). This study provides a fundamental understanding of the underlying mechanism of the rather complicated CO2 adsorption process and sheds important insights on design, synthesis, and optimization of highly efficient CO2 capture materials.

HIV-positive women with anal high-grade squamous intraepithelial lesions: a study of 153 cases with long-term anogenital surveillance.

Women living with HIV (WLHIV) are at increased risk for human papillomavirus (HPV)-associated anal cancer. Given the "field effect" of HPV pathogenesis, some recommend that anal cancer screening should be limited to WLHIV with prior genital disease. This study aimed to characterize the relationship between anal and genital disease in WLHIV in order to better inform anal cancer screening guidelines. We retrospectively studied 153 WLHIV with biopsy-proven anal high-grade squamous intraepithelial lesions (AHSIL) and long-term evaluable cervical/vaginal/vulvar histopathology. Based on the absence or presence of genital HSIL, subjects were categorized as having isolated AHSIL or multicentric HSIL. Demographics, HIV parameters and cervical/anal HPV status were recorded. Chi-square test was used for bivariate analyses. Of 153 WLHIV with AHSIL, 110 (72%) had isolated AHSIL, while 43 (28%) had multicentric HSIL (28 cervical, 16 vulvar, and 8 vaginal HSIL). The median genital surveillance was 8 years (range 1-27). Cervical HPV16/18 infection was associated with multicentric disease (P = 0.001). Overall, 53% of multicentric cases presented genital HSIL preceding AHSIL with median interval 13 years (range 2-23). Paired anal and cervical high-risk HPV results were available for 60 women within 12 months of AHSIL diagnosis: 30 (50%) had anal infection alone, while 30 (50%) had anal/cervical coinfection by 16/18 (15%), non-16/18 (13%), or different types (22%). In conclusion, WLHIV frequently develop AHSILs without pre-existing genital disease or after long latency following a genital HSIL diagnosis. Our findings support anal cancer screening for WLHIV irrespective of prior genital disease.

Two-Dimensional Anti-Van't Hoff/Le Bel Array AlB6 with High Stability, Unique Motif, Triple Dirac Cones, and Superconductivity.

We report the discovery of a rule-breaking two-dimensional aluminum boride (AlB6-ptAl-array) nanosheet with a planar tetracoordinate aluminum (ptAl) array in a tetragonal lattice by comprehensive crystal structure search, first-principles calculations, and molecular dynamics simulations. It is a brand new 2D material with a unique motif, high stability, and exotic properties. These anti-van't Hoff/Le Bel ptAl-arrays are arranged in a highly ordered way and connected by two sheets of boron rhomboidal strips above and below the array. The regular alignment and strong bonding between the constituents of this material lead to very strong mechanical strength (in-plane Young's modulus Y x = 379, Y y = 437 N/m, much larger than that of graphene, Y = 340 N/m) and high thermal stability (the framework survived simulated annealing at 2080 K for 10 ps). Additionally, electronic structure calculations indicate that it is a rare new material with triple Dirac cones, Dirac-like fermions, and node-loop features. Remarkably, this material is predicted to be a 2D phonon-mediated superconductor with Tc = 4.7 K, higher than the boiling point of liquid helium (4.2 K). Surprisingly, the Tc can be greatly enhanced up to 30 K by applying tensile strain at 12%. This is much higher than the temperature of liquid hydrogen (20.3 K). These outstanding properties may pave the way for potential applications of an AlB6-ptAl-array in nanoelectronics and nanomechanics. This work opens up a new branch of two-dimensional aluminum boride materials for exploration. The present study also opens a field of two-dimensional arrays of anti-van't Hoff/Le Bel motifs for study.

Efficient electrocatalytic reduction of carbon dioxide by metal-doped ß12-borophene monolayers.

Electrochemical reduction of CO2 to value-added chemicals and fuels shows great promise in contributing to reducing the energy crisis and environment problems. This progress has been slowed by a lack of stable, efficient and selective catalysts. In this paper, density functional theory (DFT) was used to study the catalytic performance of the first transition metal series anchored TM-Bß12 monolayers as catalysts for electrochemical reduction of CO2. The results show that the TM-Bß12 monolayer structure has excellent catalytic stability and electrocatalytic selectivity. The primary reduction product of Sc-Bß12 is CO and the overpotential is 0.45 V. The primary reduction product of the remaining metals (Ti-Zn) is CH4, where Fe-Bß12 has the minimum overpotential of 0.45 V. Therefore, these new catalytic materials are exciting. Furthermore, the underlying reaction mechanisms of CO2 reduction via the TM-Bß12 monolayers have been revealed. This work will shed insights on both experimental and theoretical studies of electroreduction of CO2.

The initial stages of melting of graphene between 4000 K and 6000 K.

Graphene and its analogues have some of the highest predicted melting points of any materials. Previous work estimated the melting temperature for freestanding graphene to be a remarkable 4510 K. However, this work relied on theoretical methods that do not accurately account for the role of bond breaking or complex bonding configurations in the melting process. Furthermore, experiments to verify these high melting points have been challenging. Practical applications of graphene and carbon nanotubes at high temperatures will require a detailed understanding of the behavior of these materials under these conditions. Therefore, we have used reliable ab initio molecular dynamics calculations to study the initial stages of melting of freestanding graphene monolayers between 4000 and 6000 K. To accommodate large defects, and for improved accuracy, we used a large 10 × 10 periodic unit cell. We find that the system can be heated up to 4500 K for 18 ps without melting, and 3-rings and short lived broken bonds (10-rings) are observed. At 4500 K, the system appears to be in a quasi-2D liquid state. At 5000 K, the system is starting to melt. During the 20 ps simulation, diffusion events are observed, leading to the creation of a 5775 defect. We calculate accurate excitation energies for these configurations, and the pair correlation function is presented. The modified Lindemann criterion was calculated. Graphene and nanotubes together with other proposed high melting point materials would be interesting candidates for experimental tests of melting in the weightless environment of space.

Halogenated MOF-5 variants show new configuration, tunable band gaps and enhanced optical response in the visible and near infrared.

Inspired by recent experimental fabrication of mono-halogenated versions of Metal-Organic Framework MOF-5 (i.e., X-MOF-5, X = F to I) and some experimentally known fully halogenated MOF compounds, we systematically studied frameworks incorporating full halogenation of the BDC linkers of the prototypical Iso-Reticular Metal-Organic Framework (IRMOF) series, exemplified by MOF-5. Using quantum chemistry calculations, we find that halogenation leads to a 90° rotation of the aryl group, which is mainly ascribed to overcrowding between halogen atoms and the carboxyl and benzene ring and strong repulsion among in-plane atoms/groups. The 90° configuration decreases the repulsion, and maximizes the stabilization energy, and is therefore more stable than 0° configuration. We find that the band gap can be tuned from 4.1 to 1.5 eV as we go from F, Cl, Br, to I. This extends the optical response of these experimentally accessible materials through the visible and infrared region. We have also considered a broader range of new materials that substitute various metals for Zn. Totally, 70 materials were systematically examined computationally including (M4O)(BDC-Z4)3 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba; Z = H, F, Cl, Br, I). For the full range of materials, we calculate band gaps of 4.2 to 1.0 eV, corresponding to a threshold of absorption of 290-1240 nm. Four selected materials were tested for stability using short 5 ps molecular dynamics simulations up to 600 K. The new materials with the smallest band gaps could potentially be used in near-infrared (NIR) light-emitting devices. Other properties, e.g., bulk moduli, formation energy, chemical bonding, and optical properties, were also investigated. The present results may provide new materials for use as novel photocatalysts, photoactive materials for photovoltaic cells, or functional devices in nanoelectronics and optoelectronics.

Adding a new dimension to the chemistry of phosphorus and arsenic.

The recently discovered phosphorenes are emerging as promising 2D materials for nanoelectronics. Novel structures and topologies can produce new properties and functionalities, and pave the way for potential new applications. For the first time, we predict two novel highly stable free-standing 2D monolayers of P and As with exotic hypercoordination motifs. This is the first hexacoordinate P and planar hexacoordinate As extended sheet. Ab initio calculations and molecular dynamics simulations demonstrate that these new Cu2X (X = P/As) materials are highly stable and are diamagnetic and metallic. Cu2P is slightly buckled without magnetism. For Cu2As, the exactly planar motif is the ground state. We observe an interesting phenomenon, i.e., buckling quenching of the magnetism in a 2D crystal. To our knowledge, this is the first example of buckling quenching of the magnetism in a 2D crystal. These results add a new dimension to the chemistry of phosphorus and arsenic.

Computational Study of Quasi-2D Liquid State in Free Standing Platinum, Silver, Gold, and Copper Monolayers.

Recently, freestanding atomically thick Fe metal patches up to 10 atoms wide have been fabricated experimentally in tiny pores in graphene. This concept can be extended conceptually to extended freestanding monolayers. We have therefore performed ab initio molecular dynamics simulations to evaluate the early melting stages of platinum, silver, gold, and copper freestanding metal monolayers. Our calculations show that all four freestanding monolayers will form quasi-2D liquid layers with significant out-of-plane motion and diffusion in the plane. Remarkably, we observe a 4% reduction in the Pt most likely bond length as the system enters the liquid state at 2400 K (and a lower effective spring constant), compared to the system at 1200 and 1800 K. We attribute this to the reduced average number of bonds per atom in the Pt liquid state. We used the highly accurate and reliable Density Functional Theory (DFT-D) method that includes dispersion corrections. These liquid states are found at temperatures of 2400 K, 1050 K, 1600 K, and 1400 K for platinum, silver, gold, and copper respectively. The pair correlation function drops in the liquid state, while the bond orientation order parameter is reduced to a lesser degree. Movies of the simulations can be viewed online (see Supplementary Material).

Glitter in a 2D monolayer.

We predict a highly stable and robust atomically thin gold monolayer with a hexagonal close packed lattice stabilized by metallic bonding with contributions from strong relativistic effects and aurophilic interactions. We have shown that the framework of the Au monolayer can survive 10 ps MD annealing simulations up to 1400 K. The framework is also able to survive large motions out of the plane. Due to the smaller number of bonds per atom in the 2D layer compared to the 3D bulk we observe significantly enhanced energy per bond (0.94 vs. 0.52 eV per bond). This is similar to the increase in bond strength going from 3D diamond to 2D graphene. It is a non-magnetic metal, and was found to be the global minima in the 2D space. Phonon dispersion calculations demonstrate high kinetic stability with no negative modes. This 2D gold monolayer corresponds to the top monolayer of the bulk Au(111) face-centered cubic lattice. The close-packed lattice maximizes the aurophilic interactions. We find that the electrons are completely delocalized in the plane and behave as 2D nearly free electron gas. We hope that the present work can inspire the experimental fabrication of novel free standing 2D metal systems.

Revealing unusual chemical bonding in planar hyper-coordinate Ni2Ge and quasi-planar Ni2Si two-dimensional crystals.

We discover unusual chemical bonding in a novel planar hyper-coordinate Ni2Ge free-standing 2D monolayer, and also in a nearly planar slightly buckled Ni2Si monolayer. This unusual bonding is revealed by Solid State Adaptive Natural Density Partitioning analysis. This analysis shows that a new type of 2c-2e Ni-Si σ and 3c-2e Ni-Ge-Ni σ bonds stabilize these 2D crystals. This is completely different from any previously known 2D crystals. Both of these free-standing monolayers are global minima in two-dimensional space. Although their exotic structure has unprecedented chemical bonding, they show extraordinary stability as single layers. The stabilities of these frameworks are confirmed by phonon dispersion calculations and ab initio molecular dynamics calculations. For Ni2Si, the framework was maintained during short 10 ps molecular dynamics annealing up to 1500 K, while Ni2Ge survived 10 ps runs up to 900 K. Both systems are predicted to be non-magnetic and metallic. As these new 2D crystals contain hypercoordinated Group 14 atoms, they are examples of a new class of 2D crystals with unconventional chemical bonding and potentially exciting new properties. Interestingly, we find that the stabilities of Ni2Si and Ni2Ge are much higher than that of silicene and germanene. Thus, this work provides a novel way to stabilize 2D sheets of Group 14 elements.

Ten new predicted covalent organic frameworks with strong optical response in the visible and near infrared.

We use density functional theory to predict and evaluate 10 novel covalent organic frameworks (COFs), labeled (X4Y)(BDC)3, (X = C/Si; Y = C, Si, Ge, Sn, and Pb), with topology based on metal organic framework isoreticular metal-organic framework (IRMOF-1), but with new elements substituted for the corner atoms. We show that these new materials are stable structures using frequency calculations. For two structures, (C4C and Si4C) molecular dynamics simulations were performed to demonstrate stability of the systems up to 600 K for 10 ps. This demonstrates the remarkable stability of these systems, some of which may be experimentally accessible. For the C4C material, we also explored the stability of isolated corners and linkers and vacuum and started to build the structure from these pieces. We discuss the equilibrium lattice parameters, formation enthalpies, electronic structures, chemical bonding, and mechanical and optical properties. The predicted bulk moduli of these COFs range from 18.9 to 23.9 GPa, larger than that of IRMOF-1 (ca. 15.4 GPa), and larger than many existing 3D COF materials. The band gaps range from 1.5 to 2.1 eV, corresponding to 600-830 nm wavelength (orange through near infrared). The negative values of the formation enthalpy suggest that they are stable and should be experimentally accessible under suitable conditions. Seven materials distort the crystal structure to a lower space group symmetry Fm-3, while three materials maintain the original Fm-3m space group symmetry. All of the new materials are highly luminescent. We hope that this work will inspire efforts for experimental synthesis of these new materials.

The new dimension of silver.

Although significant progress in the fabrication and applications of graphene-like materials has been made, free-standing metal monolayers are extremely rare due to the challenges in fabrication. Furthermore, such structures are often unstable versus 3D close-packed forms. Silver is an important noble metal with many unique properties, and has a wide range of applications in daily life and industry. Here, we display a new dimension of silver, i.e., a 2D Ag monolayer, with reduced dimensionality and quantum confinement. We observe that the Ag monolayer is stable in ab initio molecular dynamics simulations up to 800 K for 10 ps. The bond strength per atom actually increases from 0.21 eV for the bulk with twelve bonds to 0.33 eV for the 2D layer with six bonds. This increase in bond strength contributes to the stability of the free-standing 2D layer. Detailed density functional theory calculations are used to predict the properties of this material. The 2D Ag monolayer is the global minimum structure. One electron is delocalized into the whole sheet and is donated to a nearly free 2D electron gas.

Post-anti-van't Hoff-Le Bel motif in atomically thin germanium-copper alloy film.

Using quantum-chemical calculations and reduced dimensionality, we show that the "post-anti-van't Hoff-Le Bel" motif of germanium can be stabilized in a novel two-dimensional (2D) copper-germanium alloy film. This hypercoordinate sheet is the first stable planar hexacoordinate germanium material in 2D space. First principle calculations and molecular dynamics indicate that this Cu2Ge alloy film is a diamagnetic metal, and survives brief 10 ps annealing up to 1200 K. The electron delocalization and chemical bonding are unique and different from that of graphene, the all-boron α-sheet, and the BC3 honeycomb epitaxial sheet. Furthermore, we open a new route for the transformation of individual molecules to extended planar 2D sheets. This will provide new possibilities for extended planar hypercoordinate materials. We hope that these results will provide new opportunities to design post-anti-van't Hoff-Le Bel motifs by manipulation of dimensionality and quantum confinement.