Pure Monolayer Research Articles

A general method for large-area and broadband enhancing photoresponsivity in graphene photodetectors

We report on a general method for broadband responsivity enhancement in graphene photodetectors based on the sandwiched graphene structure under total internal reflection. The optical absorption is ∼25% for transverse electric waves for pure monolayer graphene, and the responsivity of pure monolayer graphene photodetectors is 0.012 A/W, which is one or two orders of magnitude larger than the normal incidence excitation. The enhanced responsivity covers a wide wavelength range from 300 to 1550 nm. Further, this method is not limited by the device, and it is a general method used to improve the light-graphene coupling, thus increasing the responsivity of graphene photodetectors largely. And this method allows large area preparation to meet the needs of the free space light detections.

Order of Dry and Wet Mixed-Length Self-Assembled Monolayers

Functionalizing the surface of metal and oxide materials with self-assembled monolayers is an elegant method for tuning the chemical properties of the surfaces. The properties of a coated surface depend on both the chemical nature of the termination as well as the order of the monolayer. One commonly used platform is alkylsilanes on silica surfaces. Here we characterize the disorder of self-assembled monolayers having a mixture of two monomer lengths using sum-frequency generation (SFG) and Fourier transform infrared spectroscopy. We find that mixed monolayer order varies smoothly as a function of composition. The mixed monolayers are more disordered than either pure monolayer, and of the pure monolayers, the shorter monolayer is more disordered. The nonlinear relationship between monolayer disorder and composition, along with atomic force microscopy images, suggests completely homogeneous mixing or very small domains of the monomers. The order of the monolayers as determined by SFG spectroscopy does not ...

Tunable electronic and magnetic properties of a MoS2 monolayer with vacancies under elastic planar strain: Ab initio study

Electronic and magnetic properties of a molybdenum disulfide (MoS2) monolayer with some intrinsic and extrinsic vacancies are investigated using ab initio method in the presence of planar strain distributions. The calculations are carried out within the density functional theory (DFT) as implemented in SIESTA package. By using fully relaxed structures and applying a full spin-polarized description to the system, we concentrate on created magnetic moment due to the vacancies under different planar strains. The results show that the extrinsic MoS6 vacancy induces a net magnetic moment of 6.00μB per supercell. Also, it is found that the pure MoS2 monolayer for the most cases does not show any magnetic properties under the planar strain. While the net magnetic moment of MoS2 monolayer with the vacancies enhances as the planar tensile strain is applied. The tunable magnetic moment of MoS2 monolayer may be utilized for the development of spintronic and flexible electronic nano-devices.

CoenzymeQ10 localizations in model membranes. A Langmuir monolayer study

The interaction of coenzyme Q10 (CoQ10) in a monolayer of 1,2-dipalmitoyl-sn-glysero-3-phospho-L-choline (DPPC), in a monolayer of 1,2-dierucoyl-sn-glysero-3-phospho-L-choline (DEPC), in a monolayer of 1-palmitoyl-2-oleoyl-sn-glysero-3-phospho-L-serine (POPS) and in a monolayer of total lipid extract from pig brain (PB) has been investigated by using the Langmuir monolayer technique. Surface pressure (π)-mean molecular area (mma) isotherms have been measured for pure lipid monolayers and lipid monolayers with 0.5, 1.0, 2.0, 5.0 and 10.0 mol% CoQ10 concentrations. At the biological concentration (1.0-3.0 mol%) of CoQ10, intercalation of CoQ10 occurs in the lipid acyl chains of DPPC, POPS and PB monolayers. Above the biological concentration of CoQ10, the CoQ10 molecule induces domain formation in the monolayers of DPPC, POPS and PB lipids. The DEPC monolayer behavior deviates from the other lipids in this study. At 2.0 mol% the CoQ10 promotes very dense lipid packing, and the CoQ10 molecule is located parallel to the DEPC acyl chains at all concentrations.

Structural Transition in Layered As(1-x)P(x) Compounds: A Computational Study.

As a way to further improve the electronic properties of group V layered semiconductors, we propose to form in-layer 2D heterostructures of black phosphorus and gray arsenic. We use ab initio density functional theory to optimize the geometry, determine the electronic structure, and identify the most stable allotropes as a function of composition. Because pure black phosphorus and pure gray arsenic monolayers differ in their equilibrium structure, we predict a structural transition and a change in frontier states, including a change from a direct-gap to an indirect-gap semiconductor, with changing composition.

Detailed Analysis of the Surface Area and Elasticity in the Saturated 1,2-Diacylphosphatidylcholine/Cholesterol Binary Monolayer System.

The surface pressure-area (π-A) isotherms of DMPC, DPPC, and DSPC/cholesterol binary monolayers were systematically measured with great care to gain insight into the lateral molecular packing in these binary monolayer systems. The average molecular area A and the area elastic modulus C(s)⁻¹ at a given surface pressure were calculated as a function of cholesterol mole fraction x(chol). As a result, data reliable enough for the analysis of detailed phase behavior were obtained. We identified several characteristic phase regions and assigned the phase state in each region on the basis of the deviation of A(x(chol)) and C(s)⁻¹(x(chol)) from ideal additivity. We also estimated the partial molecular areas of DMPC, DPPC, DSPC, and cholesterol in the single-phase regions, where C(s)⁻¹(x(chol)) values fell on an ideal additivity curve. We found that the addition of cholesterol induces the formation of a highly condensed phase where the diacylphosphatidylcholine (diacyl PC) molecule has a surface area even smaller than that in the solid phase, irrespective of the surface pressure and the chain length of diacyl PC. Here, we call the cholesterol-induced condensed phase the CC phase. Furthermore, we demonstrated that the basic features of A(x(chol)) and C(s)⁻¹(x(chol)) profiles can be explained semiquantitatively by assuming the state of vicinity lipids surrounding sparsely distributed cholesterol molecules in the low x(chol) region as a third state of the diacyl PC molecule in addition to the states in the pure diacyl PC monolayer and in the CC phase.

Self-Assembled p-Carborane Analogue of p-Mercaptobenzoic Acid on Au{111}

The p-carborane cluster analogue of p-mercaptobenzoic acid, 1-HS-12-COOH-1,12-C2B10H10, has been synthesized and characterized using nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction analysis, quantum-chemical calculations, and scanning tunneling microscopy. The single-crystal structure and selected packing aspects are discussed and presented in comparison with the two-dimensional periodic arrangements. Scanning tunneling micrographs, recorded under ambient conditions, are used to compare pure monolayers of 1-HS-1,12-C2B10H11 to coadsorbed monolayers of both the parental precursor and carboxyl-functionalized p-carboranethiolate on Au{111}. Monolayers of both constituents are further characterized by X-ray photoelectron spectroscopy, which shows good agreement between the stoichiometry of each pure monolayer and the nominal stoichiometries of the respective molecules. Results indicate that most of the molecules of both derivatives adsorb as thiolates but that a small fraction of eac...

Impact of Lipid Oxidization on Vertical Structures and Electrostatics of Phospholipid Monolayers Revealed by Combination of Specular X-ray Reflectivity and Grazing-Incidence X-ray Fluorescence.

The influence of phospholipid oxidization of floating monolayers on the structure perpendicular to the global plane and on the density profiles of ions near the lipid monolayer has been investigated by a combination of grazing incidence X-ray fluorescence (GIXF) and specular X-ray reflectivity (XRR). Systematic variation of the composition of the floating monolayers unravels changes in the thickness, roughness and electron density of the lipid monolayers as a function of molar fraction of oxidized phospholipids. Simultaneous GIXF measurements enable one to qualitatively determine the element-specific density profiles of monovalent (K(+) or Cs(+)) and divalent ions (Ca(2+)) in the vicinity of the interface in the presence and absence of two types of oxidized phospholipids (PazePC and PoxnoPC) with high spatial accuracy (±5 Å). We found the condensation of Ca(2+) near carboxylated PazePC was more pronounced compared to PoxnoPC with an aldehyde group. In contrast, the condensation of monovalent ions could hardly be detected even for pure oxidized phospholipid monolayers. Moreover, pure phospholipid monolayers exhibited almost no ion specific condensation near the interface. The quantitative studies with well-defined floating monolayers revealed how the elevation of lipid oxidization level alters the structures and functions of cell membranes.

Interfacial stabilization of the antitumoral drug Paclitaxel in monolayers of GM1 and GD1a gangliosides

Molecular interactions between the anti-cancer agent Paclitaxel (Ptx), and two gangliosides with different sialic acid content, GM1 and GD1a, were investigated using the Langmuir film balance technique. Ptx showed interfacial activity reducing the air/water surface tension by 18mN·m−1. However, the drug was able to insert into preformed ganglioside monolayers at much higher surface pressures, indicating a preferential interaction of Ptx with GM1 and GD1a. Compression isotherms of binary mixtures of Ptx and GM1 or GD1a also indicated non-ideal mixed monolayers in which the drug became stabilized at the interface in the presence of gangliosides. Ptx reached much higher surface pressure values in the mixed monolayers than those sustained in pure Ptx, although partial desorption of the drug from the interface into the subphase was also observed at high Ptx contents. The mean molecular area of the mixtures showed condensation, mainly in the case of GD1a, whereas Ptx induced a decrease in the compressibility of monolayers when mixed with either GM1 or GD1a. Additionally, Brewster angle microscopy analysis indicated that higher amounts of Ptx are present at the mixed ganglioside/Ptx interface when compared to pure drug monolayers. Finally, GD1a micelles increased in size in the presence of Ptx, whereas GM1 micelles kept their diameter, according to dynamic light scattering measurements, which could be explained by the different properties of ganglioside monolayers. The results obtained on ganglioside–Ptx interactions allowed interpreting the different Ptx loading capacity of GM1 and GD1a, enabling them to act as potential drug carriers.

Theoretical Study of Graphene Doping Mechanism by Iodine Molecules

The adsorption of iodine atoms and molecules on graphene is studied in detail, using first-principles calculations that include nonlocal correlation effects by means of van der Waals density functional approach. Structural, energetic, and electronic structure properties of these systems are reported. We demonstrate that graphene surface can be doped by atomic and molecular iodine. An upward shift of the Dirac point from the Fermi level with values of 0.45 and 0.08 eV is observed for adsorbed atoms and adsorbed I2, respectively. It corresponds to graphene hole densities to be around 1.2 × 1013–3.9 × 1011 cm–2. We also show that the iodine molecule does not dissociate in contact with pure graphene monolayer. Calculation of the surface free energy reveals that the orientation of the adsorbed iodine molecules crucially depends on its concentration and the system temperature. The corresponding phase diagram indicates that the in-plan orientation of molecules is more stable when the iodine concentration decreas...

A novel optical property induced by MO, S vacancy and V-doped in monolayer MoS2

The electronic structure and optical properties of Mo, S vacancy and V doping in MoS2 monolayer will be investigated through first-principles calculations based on the density functional theory. The results indicate that the MoS2 with Mo, S vacancy and V doping (Mo14VS32, Mo15VS31 and Mo14VS31) will gain the property of magnetic semiconductor with the magnetic moment of 1μB, 1μB and 0.95μB, respectively. The optical properties of these V-doped and vacancy defect structures all reflect the phenomenon of red shift. The absorption edge of pure monolayer molybdenum disulfide is 0.8eV, whereas the absorption edges of Mo14VS32, Mo15VS31 and Mo14VS31 become 0eV, 0.2eV and 0.16eV, respectively. As a potential material, MoS2 is widely used in many fields such as the production of optoelectronic devices, military devices and civil devices.

Stability and softening of a lipid monolayer in the presence of a pain-killer drug

The aim of this study is to investigate the interaction of a drug (Piroxicam, 4-hydroxy-2-methyl-N-(2-pyridinyl)-2H-1,2-benzothiazine-3-carboxamide 1,1 dioxide) with a lipid (DMPC) monolayer used as a membrane-mime in terms of drug-induced changes in stability and compressibility with variation in temperature, surface-pressure, drug-dose and ionic states of the monolayers. Drug-induced fluidization is noticed in the π−A isotherms through increase in phase-transition pressure at constant temperature. The long-term dynamics of the lipid-monolayer is characterized by algebraic decays in surface-energy E with time t, E∼t−p1,2,3, with an initial decay exponent p1 that changes to p2 after ∼1000s, and, at high surface pressures and/or drug-dose, to a third exponent p3 after ∼3500s, suggesting structural reorganizations in the monolayer. With increasing drug–lipid ratio (D/L), p1 shows a decrease ending at an almost constant value after 0.05, p2 shows an almost negligible lowering while p3 shows a monotonic and considerable increase. The reorganization is summarized by proposing two mechanisms: (a) ‘charging–discharging’ where drug-molecules sitting parallel to the interface increase headgroup separations and (b) ‘discharging–charging’ where drug-molecules sitting roughly perpendicular to the interface bring headgroups closer. Drug-induced softening of lipid-monolayers is characterized by the compressibilites of pure and mixed lipid monolayers. Compressibility-change (i.e., compressibility difference between drug/lipid and pure lipid monolayer) with pressure is maximum in the LE–LC transition zone and compressibility-change with drug-dose reveals an optimum dose of drug for maximum increase in compressibility. Molecular dynamics simulation shows that the ordering in the different parts of the lipid chains is changed to different extents in the presence of drugs with maximum change near the headgroups and again points to an optimum dose for maximum disorder.

Phosphatidylcholine-fatty Alcohols Equilibria in Monolayers at the Air/Water Interface.

Monolayers of phosphatidylcholine (PC), tetradecanol (TD), hexadecanol (HD), octadecanol (OD) and eicosanol (E) and their binary mixtures were investigated at the air/water interface. The surface tension values of pure and mixed monolayers were used to calculate π–A isotherms. The surface tension measurements were carried out at 22 °C using a Teflon trough and a Nima 9000 tensiometer. The interactions between phosphatidylcholine and fatty alcohols (tetradecanol, hexadecanol, octadecanol, eicosanol) result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants, Gibbs free energy values and areas occupied by one molecules of PC–TD, PC–HD, PC–OD and PC–E complexes. We considered the equilibrium between the individual components and the complex and established that phosphatidylcholine and fatty alcohols formed highly stable 1:1 complexes.

Mapping the structure-activity relationship of β-sitosteryl fatty acid esters in condensing phospholipid monolayers

The phase behavior of twelve synthesized β-sitosteryl fatty acid esters with acyl moieties with different chain lengths (C2:0–C18:0) and different degrees of unsaturation (C18:1–C18:3) were investigated in pure and mixed Langmuir monolayers with phospholipids. The surface-pressure isotherms showed that short chain β-sitosteryl fatty acid esters gave smaller mean molecular areas and had decreased monolayer stability and the long chain steryl esters did not produce collapsed plateaus. All the steryl esters displayed strong condensing effects, but there was a pronounced structural dependency: medium chain esters (C8 and C10) were less efficient than short and long chain esters. Atomic force microscopy imaging demonstrated that monolayers mixed with dipalmitoyl phosphatidylcholine (DPPC) displayed both DPPC-rich and steryl lipid-rich domains. However, the height and area differences between the two phases and the roughness and morphologic patterns were very dependent on the steryl lipid concentrations as well as the length, the degree of unsaturation and the molecular conformations of the acyl segments. These findings not only provide a better understanding of the interactions between phytosteryl hydrophobic derivatives and biomembranes, but also may be of general use for the design and engineering of phytosterol structural derivations for specific food and pharmaceutical applications. Open image in new window

Pure and Oxidized Ag Substrate Influence on the Phase Transformation and Semiconducting Behaviors of Layered ZnO: A First-Principles Study

Graphitic-like ZnO layers have been experimentally synthesized on metal substrates over the past few years. Nevertheless, the impact of metal substrates on the structural and electric properties of ZnO is still unclear. Utilizing first-principle calculations with van der Waals correction, we found that the phase transformation from graphitic-like to wurtzite structure occurs when the thickness of freestanding ZnO exceeds seven layers. With the presence of pure Ag(111) substrate, the critical transformation thickness decreases to two layers because of the depolarization effect originating from the charge transfer from Ag substrate to ZnO. Band structure analysis displays the semiconducting behaviors for the freestanding graphitic-like ZnO layers. On the pure Ag substrate, monolayer and bilayer ZnO is n-doped by the substrate and a metallic character of ZnO is observed. Importantly, the semiconducting behavior of ZnO layers is maintained when ZnO is in contact with oxidized Ag substrate because of less char...

Affinity separation by Langmuir–Blodgett deposition of bovine serum albumin using arachidic acid as specific ligand

Affinity separation by Langmuir–Blodgett (LB) deposition is a high resolution bioseparation procedure that takes advantage of the affinity between biomolecules and surface active materials, inducing the formation of a Langmuir-biofilm (L-biofilm) which is later removed from the air–water interface by deposition onto a solid substrate. This novel method is here applied to the separation of bovine serum albumin (BSA) by using arachidic acid (AA) as a ligand. Optimization of the experimental procedure choosing those conditions which favor Y-type multilayer deposition and low protein solubility is key. Both goals are achieved simultaneously by using Cd2+ as a counterion and a pH equal to the counterion pKA, that is pH=5.6. The selection of a target area of 22Å2/molecule, a value close to the footprint of AA in solid state, ensures that the protein will be squeezed out of the interface and bounded to the ligand. The transfer ratio (TR) values obtained during depositions for up to 6 layers show that the deposition was successful, and spectrophotometric analysis show that a 48% of the protein initially present at the subphase is removed. Additionally, the efficiency of separation for each layer is close to 100% with respect to the maximum amount of BSA that could fit in the film, assuming that the molecule stands vertically in the L-biofilm structure (long axis perpendicular to the solid). Atomic Force Microscopy (AFM) analysis qualitatively confirms the presence of the BSA in the deposited biofilm. The images illustrate that the presence of BSA strongly affects the topography of the surface, in the form of roughness and protrusions, unlike a pure arachidic acid monolayer. Roughness clearly increases when BSA is present in the deposited film. In summary, experimental evidence indicates the high potential of LB as a method for purification of proteins.

Broadband photodetectors based on graphene-Bi2Te3 heterostructure.

Recently, research on graphene based photodetectors has drawn substantial attention due to ultrafast and broadband photoresponse of graphene. However, they usually have low responsivity and low photoconductive gain induced by the gapless nature of graphene, which greatly limit their applications. The synergetic integration of graphene with other two-dimensional (2D) materials to form van der Waals heterostructure is a very promising approach to overcome these shortcomings. Here we report the growth of graphene-Bi2Te3 heterostructure where Bi2Te3 is a small bandgap material from topological insulator family with a similar hexagonal symmetry to graphene. Because of the effective photocarrier generation and transfer at the interface between graphene and Bi2Te3, the device photocurrent can be effectively enhanced without sacrificing the detecting spectral width. Our results show that the graphene-Bi2Te3 photodetector has much higher photoresponsivity (35 AW(-1) at a wavelength of 532 nm) and higher sensitivity (photoconductive gain up to 83), as compared to the pure monolayer graphene-based devices. More interestingly, the detection wavelength range of our device is further expanded to near-infrared (980 nm) and telecommunication band (1550 nm), which is not observed on the devices based on heterostructures of graphene and transition metal dichalcogenides.

Modulation of Phosphoinositide Monolayer Compressibilities by Physiological Levels of Ca2+

Phosphoinositides (PIs) play a crucial role in many cellular processes that occur at the plasma membrane such as exocytosis and endocytosis. These processes not only locally enrich the membrane with PIs and are often accompanied by intracellular calcium release, but they also involve mechanical membrane deformations. Thus, the question arises how mechanical properties such as compressibility of highly negatively charged lipids such as phosphatidylinositol bisphosphate (PIP2) are modulated by physiological levels (0-1000nM) of Ca2+ ions. Using pure monolayers and a Langmuir film balance, we investigated the effect of bivalent ions on the compressibility of Phosphatidylinositol (PI), Phosphatidylinositol 4,5-bisphosphate (PIP2), 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DOPG)and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). In addition, we also present a theoretical framework that describes the relationship between electrical surface potentials and compressibilities which shows good agreement with our experimental findings.

Single layer lead iodide: computational exploration of structural, electronic and optical properties, strain induced band modulation and the role of spin-orbital-coupling.

Graphitic like layered materials exhibit intriguing electronic structures and thus the search for new types of two-dimensional (2D) monolayer materials is of great interest for developing novel nano-devices. By using density functional theory (DFT) method, here we for the first time investigate the structure, stability, electronic and optical properties of monolayer lead iodide (PbI2). The stability of PbI2 monolayer is first confirmed by phonon dispersion calculation. Compared to the calculation using generalized gradient approximation, screened hybrid functional and spin-orbit coupling effects can not only predicts an accurate bandgap (2.63 eV), but also the correct position of valence and conduction band edges. The biaxial strain can tune its bandgap size in a wide range from 1 eV to 3 eV, which can be understood by the strain induced uniformly change of electric field between Pb and I atomic layer. The calculated imaginary part of the dielectric function of 2D graphene/PbI2 van der Waals type hetero-structure shows significant red shift of absorption edge compared to that of a pure monolayer PbI2. Our findings highlight a new interesting 2D material with potential applications in nanoelectronics and optoelectronics.

Silicon sensitisation using light harvesting layers

Langmuir–Blodgett monolayers of a cyanine dye mixed with stearic acid were deposited on glass and silicon substrates with spacer layers of pure stearic acid monolayers or silicon dioxide films deposited by PECVD. By using the time correlated single photon counting technique, time resolved emission spectra (TRES) and decay curves were measured to characterise the dependence of energy transfer rate on the separation between the dye monolayer and the silicon surface and also for the dye concentrations in the monolayers. We observe interlayer energy transfer between monomers, dimers and higher aggregates present in the monolayer deposited on glass but also competing directly with energy transfer to silicon at close distances. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when deposited close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for monolayers deposited at distances close to the silicon surface.