Patent Application
20240059572
The present invention relates generally to the material science, and more particularly to borophane polymorphs and synthesis methods of same.
The background description provided herein is for the purpose of generally presenting the context of the invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the invention.
Since its initial experimental realization, studies of borophene have focused on its diverse polymorphism and predicted properties that include two-dimensional (2D) anisotropic metallicity, high mechanical strength and flexibility, and phonon-mediated superconductivity. However, borophene rapidly oxidizes in air, which has confined experimental characterization to ultrahigh vacuum (UHV) conditions and also posed challenges to integrating borophene into practical devices. Chemical passivation can suppress ambient oxidation for electronic materials. For example, monohydride termination of the dangling bonds on silicon surfaces minimizes native oxide formation. Similarly, covalent modification of 2D black phosphorus improves morphological stability and preserves electronic properties in ambient conditions. First principles calculations have suggested that borophene can also be stabilized through surface hydrogenation.
Beyond passivation, chemical functionalization can modulate the electronic properties of 2D materials. For instance, converting the carbon bonding configuration from sp2 in graphene to sp3, hydrogenated graphene (i.e., ‘graphane’) leads to a tunable bandgap based on the hydrogen surface concentration. Drawing inspiration from graphane, hydrogenated borophene (i.e., ‘borophane’) has been explored theoretically. Predicted electronic properties include metallic, semiconducting, and Dirac characteristics with ultrahigh Fermi velocity and thermal conductance. Although hydrogen boride nanosheets have been reported based on chemical reactions involving inorganic salts, atomically well-defined synthesis and characterization of borophane polymorphs have not yet been achieved.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.
In one aspect, the invention relates to a method of synthesizing borophane polymorphs. The method in one embodiment comprises growing borophene on a substrate in an ultrahigh vacuum (UHV) chamber; and performing hydrogenation of the borophene in situ to obtain a diverse set of borophane polymorphs.
In one embodiment, the UHV chamber has a base pressure of about 1.0×10−10 mbar during the borophene growth.
In one embodiment, the substrate comprises a substrate having a metal film formed of Ag, Au, Cu, Al, or Ir.
In one embodiment, the substrate comprises an atomically clean Ag(111) substrate having about 480-720 nm thick Ag(111) on a mica substrate.
In one embodiment, the substrate is obtained by cleaning the Ag(111) substrate via repeated cycles of Ar+ sputtering followed by annealing at about 450-550° C.
In one embodiment, said growing the borophene on the substrate comprises depositing boron on the substrate at a substrate temperature.
In one embodiment, said depositing the boron is performed by electron-beam evaporation of a solid boron rod, or high temperature effusion of the solid boron rod in a high temperature effusion cell.
In one embodiment, the solid boron rod has a purity of about 99.999-99.99999% boron.
In one embodiment, the borophene grown on the substrate has a dominant borophene polymorph determined by the substrate temperature.
In one embodiment, the substrate temperature is at about 320-660° C. during boron deposition.
In one embodiment, said hydrogenation is performed in situ by exposing the borophene to atomic hydrogen.
In one embodiment, the atomic hydrogen is generated by dissociation of the molecular hydrogen with a tungsten filament, or a platinum filament, or an iridium filament, or platinum/iridium filament, or by a hydrogen atom beam source that thermally crack molecular hydrogen into atomic hydrogen.
In one embodiment, during hydrogenation, the borophene is maintained at room temperature and directly faced the tungsten filament with a distance of about 8-12 cm.
In one embodiment, the chamber pressure during hydrogenation is maintained at about 1.0×10−7-5.0×10−6 mbar for about 1-20 min.
In another aspect of the invention, the method of synthesizing borophane polymorphs, comprises hydrogenating borophene with atomic hydrogen in ultrahigh vacuum (UHV).
In one embodiment, the borophene is grown on a substrate in the UHV by elemental boron evaporation.
In one embodiment, the substrate comprises a substrate having a metal film formed of Ag, Au, Cu, Al, or Ir.
In one embodiment, the borophene grown on the substrate has a dominant borophene polymorph determined by a substrate temperature of the substrate.
In one embodiment, the substrate temperature is at about 320-660° C. during boron deposition.
In one embodiment, said hydrogenating the borophene is performed by exposing borophene to atomic hydrogen.
In one embodiment, the atomic hydrogen is generated by dissociation of molecular hydrogen with a tungsten filament, or a platinum filament, or an iridium filament, or platinum/iridium filament, or by a hydrogen atom beam source that thermally crack molecular hydrogen into atomic hydrogen.
In yet another aspect, the invention relates to borophane polymorphs being synthesized according to the above disclosed method.
In one embodiment, the borophane polymorphs comprises at least eight borophane polymorphs.
In a further aspect, the invention relates to borophane comprising hydrogenated borophene having a diverse set of borophane polymorphs.
In one embodiment, the diverse set of borophane polymorphs comprises v1/5 borophane with square H and honeycomb H patterns, v1/5-30° borophane with staggered rectangular H and zigzag H patterns, v1/6 borophane with hexagonal H and disordered rectangular H patterns, and v1/6-30° borophane with hexagonal H and rectangular H patterns, wherein H represents hydrogen atoms.
In one embodiment, the v1/6-30° borophane with the rectangular H pattern (rect-v1/6-30° borophane) includes two-center-two-electron (2c2e) boron-hydrogen (B—H) bonds and three-center-two-electron (3c2e) boron-hydrogen-boron (B—H—B) bonds.
In one embodiment, the rect-v1/6-30° borophane has a lower work function than v1/6-30° borophene.
In one embodiment, the borophane polymorphs have tunable stoichiometric ratios of boron and hydrogen.
In one embodiment, the borophane polymorphs have negligible oxidation for multiple days following ambient exposure.
In one embodiment, the borophane polymorphs are metallic with modified local work functions compared to pristine borophene.
In one embodiment, the borophane polymorphs can be reversibly returned to pristine borophene via thermal desorption of hydrogen.
These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the invention.
The accompanying drawings illustrate one or more embodiments of the invention and together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.
The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this specification will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout.
The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms that are used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the invention. For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term are the same, in the same context, whether or not it is highlighted. It will be appreciated that same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.
It will be understood that, as used in the description herein and throughout the claims that follow, the meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates otherwise. Also, it will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the invention.
Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element's relationship to another element as illustrated in the figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in one of the figures. is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can, therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below.
It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” or “has” and/or “having”, or “carry” and/or “carrying,” or “contain” and/or “containing,” or “involve” and/or “involving, and the like are to be open-ended, i.e., to mean including but not limited to. When used in this specification, they specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
As used in this specification, “around”, “about”, “approximately” or “substantially” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about”, “approximately” or “substantially” can be inferred if not expressly stated.
As used in this specification, the phrase “at least one of A, B, and C” should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
The description below is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. The broad teachings of the invention can be implemented in a variety of forms. Therefore, while this invention includes particular examples, the true scope of the invention should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. It should be understood that one or more steps within a method may be executed in a different order (or concurrently) without altering the principles of the invention.
Borophene refers to the family of synthetic two-dimensional (2D) polymorphs of boron, which has attracted significant attention due to anisotropic metallicity, correlated electron phenomena, and diverse superlattice structures. However, borophene rapidly oxidizes when exposed to air, which has confined experimental characterization to ultrahigh vacuum (UHV) conditions, thus motivating chemical passivation approaches. Towards this end, this invention achieves covalent hydrogenation of borophene (i.e., ‘borophane’) through exposure to atomic hydrogen in UHV, resulting in a diverse set of borophane polymorphs. Borophane polymorphs are metallic with modified local work functions compared to pristine borophene, with the most prevalent borophane polymorph found to include two-center-two-electron (2c2e) boron-hydrogen (B—H) bonds and three-center-two-electron (3c2e) boron-hydrogen-boron (B—H—B) bonds. Hydrogenation acts as a robust passivation scheme for borophene with multiple borophane polymorphs suppressing measurable oxidation for several days following ambient exposure. Since this hydrogenation can be reversibly removed through thermal annealing, borophane can be returned to pristine borophene following ambient processing, thus presenting significant flexibility in borophene sample preparation. Overall, this invention reveals the richness of borophane chemistry, which is likely to inspire and motivate further exploration of covalently modified borophene polymorphs.
In one aspect, the invention relates to a method of synthesizing borophane polymorphs. The method in one embodiment comprises growing borophene on a substrate in an ultrahigh vacuum (UHV) chamber; and performing hydrogenation of the borophene in situ to obtain a diverse set of borophane polymorphs. Hydrogenation of borophene results in the synthesis of ‘borophane’ polymorphs, which possess modified local work functions and improved chemical passivation characteristics in ambient conditions.
In some embodiments, the UHV chamber has a base pressure of about 1.0×10−10 mbar during the borophene growth.
In some embodiments, the substrate comprises a substrate having a metal film formed of Ag, Au, Cu, Al, or Ir.
In some embodiments, the substrate comprises an atomically clean Ag(111) substrate having about 480-720 nm thick Ag(111) on a mica substrate.
In some embodiments, the substrate is obtained by cleaning the mica substrate via repeated cycles of Ar+ sputtering followed by annealing at about 450-550° C.
In some embodiments, said growing the borophene on the substrate comprises depositing boron on the substrate at a substrate temperature.
In some embodiments, said depositing the boron is performed by electron-beam evaporation of a solid boron rod, or high temperature effusion of the solid boron rod in a high temperature effusion cell.
In some embodiments, the solid boron rod has a purity of about 99.999-99.99999% boron.
In some embodiments, the borophene grown on the substrate has a dominant borophene polymorph determined by a substrate temperature.
In some embodiments, the substrate temperature is at about 320-660° C. during boron deposition.
In some embodiments, said hydrogenation is performed in situ by exposing the borophene to atomic hydrogen.
In some embodiments, the atomic hydrogen is generated by dissociation of the molecular hydrogen with a tungsten filament, or a platinum filament, or an iridium filament, or platinum/iridium filament, or by a hydrogen atom beam source that thermally crack molecular hydrogen into atomic hydrogen.
In some embodiments, during hydrogenation, the borophene is maintained at room temperature and directly faced the tungsten filament with a distance of about 8-12 cm.
In some embodiments, the chamber pressure during hydrogenation is maintained at about 1.0×10−7-5.0×10−6 mbar for about 1-20 min.
In another aspect, the invention relates to borophane polymorphs being synthesized according to the above disclosed method.
In some embodiments, the borophane polymorphs are metallic with modified local work functions compared to pristine borophene.
In some embodiments, the borophane polymorphs can be reversibly returned to pristine borophene via thermal desorption of hydrogen.
In some embodiments, the borophane polymorphs comprises at least eight borophane polymorphs.
In some embodiments, the borophane polymorphs comprises v1/5 borophane with square H and honeycomb H patterns, v1/5-30° borophane with staggered rectangular H and zigzag H patterns, v1/6 borophane with hexagonal H and disordered rectangular H patterns, and v1/6-30° borophane with hexagonal H and rectangular H patterns, wherein H represents hydrogen atoms.
In some embodiments, the v1/6-30° borophane with the rectangular H pattern (rect-v1/6-30° borophane) includes two-center-two-electron (2c2e) boron-hydrogen (B—H) bonds and three-center-two-electron (3c2e) boron-hydrogen-boron (B—H—B) bonds.
In some embodiments, the rect-v1/6-30° borophane has a lower work function than v1/6-30° borophene.
In some embodiments, the borophane polymorphs have tunable stoichiometric ratios of boron and hydrogen.
In some embodiments, the borophane polymorphs have negligible oxidation for multiple days following ambient exposure.
In some embodiments, the borophane polymorphs are metallic with modified local work functions compared to pristine borophene.
In some embodiments, the borophane polymorphs can be reversibly returned to pristine borophene via thermal desorption of hydrogen.
Borophene has been intensely explored due to its unique properties including 2D anisotropic metallicity, high mechanical strength and flexibility, and phonon-mediated superconductivity. However, borophene rapidly oxidizes when exposed to air, which poses significant challenges for its practical implementation. The hydrogen passivation scheme demonstrated in this invention offers an opportunity to integrate borophene into practical devices that are relevant to the fields of electronics, optoelectronics, sensing, energy-harvesting, quantum information, and related technologies. The invention may have widespread applications in the fields including, but are not limited to, optoelectronics, high frequency logic, sensing, medical imaging, energy conversion and storage, and quantum information technologies.
This invention discloses the covalent chemical functionalization and passivation of borophene for the first time. Among other things, the invention provides a number of advantages.
The hydrogenation of borophene can be viewed as a reversible passivation scheme in which the hydrogenation can be removed to regain pristine borophene once ambient processing is complete and/or robust encapsulation layers are applied.
These and other aspects of the invention are further described below. Without intent to limit the scope of the invention, exemplary instruments, apparatus, methods, and their related results according to the embodiments of the invention are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain theories are proposed and disclosed herein; however, in no way they, whether they are right or wrong, should limit the scope of the invention so long as the invention is practiced according to the invention without regard for any particular theory or scheme of action.
Borophene refers to the family of synthetic two-dimensional polymorphs of boron, which has attracted significant attention due to anisotropic metallicity, correlated electron phenomena, and diverse superlattice structures. While borophene heterostructures have recently been realized, ordered chemical modification of borophene has not yet been reported. In this exemplary study, borophane polymorphs were synthesized by hydrogenating borophene with atomic hydrogen in ultrahigh vacuum. Through atomic-scale imaging, spectroscopy, and first principles calculations, the most prevalent borophane polymorph is shown to possess a combination of two-center-two-electron (2c2e) boron-hydrogen (B—H) and three-center-two-electron (3c2e) boron-hydrogen-boron (B—H—B) bonds. Borophane polymorphs are metallic with modified local work functions that can be reversibly returned to pristine borophene via thermal desorption of hydrogen. Hydrogenation also provides chemical passivation as borophane is shown to suppress measurable oxidation for multiple days following ambient exposure.
In this exemplary example, hydrogenation of borophene is achieved by exposing borophene to atomic hydrogen in ultrahigh vacuum. Similar to the high degree of polymorphism in borophene, eight different borophane polymorphs are observed. Since it can achieve high surface coverage and possesses a highly ordered structure, rectangular v1/6-30° borophane (abbreviated as rect-v1/6-30°) is explored thoroughly to reveal its bonding structure and properties. In particular, by combining scanning tunneling microscopy/spectroscopy (STM/STS), inelastic electron tunneling spectroscopy (JETS), and density functional theory (DFT), the bonding in rect-v1/6-30° borophane is found to include two-center-two-electron (2c2e) B—H bonds and three-center-two-electron (3c2e) B—H—B bonds. In situ local work function measurements support theoretical predictions that rect-v1/6-30° borophane has a lower work function than v1/6-30° borophene. In addition, unlike pristine borophene that oxidizes almost instantaneously in ambient conditions, borophane shows negligible oxidation according to X-ray photoelectron spectroscopy (XPS) even after 1 week of ambient exposure.
Borophene was grown on atomically clean Ag(111) substrates in UHV by elemental boron evaporation with the dominant borophene polymorph determined by the substrate temperature. Subsequent hydrogenation was performed in situ by exposing the borophene sample to atomic hydrogen that was generated by cracking molecular hydrogen with a hot tungsten filament. Following this procedure, STM revealed bright protrusions on the borophene surface, which were attributed to hydrogen adatoms, as shown in
Additional borophane structures are observed following the hydrogenation of v1/6 borophene. In this case, v1/6 borophane is characterized with both hexagonal and rectangular H patterns. The hexagonal H pattern is most prevalent for v1/6 borophane with a unit cell of 0.34±0.02 nm by 0.35±0.02 nm, as shown in panel H of
High-resolution CO-STM images of rect-v1/6-30° borophane acquired at different scanning conditions are shown in panels A-B of
The simulated STM images from rect-1Htop (panel I of
STS was also performed on rect-v1/6-30° borophane to provide additional experimental evidence in support of the rect-2H structure. In particular, the differential tunneling conductance spectrum taken on rect-v1/6-30° borophane shows a distinct peak at ˜1.2 V (panel L of
The bonding geometry of rect-v1/6-30° borophane was further investigated by LETS measurements. Due to the sensitivity of vibrational spectra to isotopic identity, hydrogenated and deuterated rect-v1/6-30° borophane samples were prepared for LETS characterization. The LETS spectra of rect-v1/6-30° borophane with hydrogen (red) and deuterium (orange) in addition to pristine v1/6-30° borophene (purple) are shown in panel N of
The local work function (LWF) of rect-v1/6-30° borophane was probed with STM by measuring field-emission resonances (FERs), also known as Gundlach oscillations, which arise in the Fowler-Nordheim regime through standing-wave states in the tip-sample gap. Panel A of
To better understand the surface potential for rect-v1/6-30° borophane, DFT calculations with dipole correction were performed to simulate electrostatic potential profiles. Panel B of
The LWFs for Ag(111), v1/6-30° borophene, and rect-v1/6-30° borophane were extracted from the FER spectra as 4.34±0.11 eV, 4.58±0.12 eV, and 4.36±0.09 eV, respectively (
The ambient stability of borophane was evaluated using XPS and atomic force microscopy (AFM) measurements following different periods of ambient exposure. Unencapsulated borophene sheets chemically degrade in ambient conditions. We confirmed this result by performing XPS on unencapsulated mixed-phase v1/5/v1/6-30° borophene following 1 hour of ambient exposure at 20° C., relative humidity between 20%-50% (panel A of
In contrast, the borophane sample exhibits markedly improved chemical and morphological stability in ambient conditions. It should be noted that this sample contains multiple borophane polymorphs (e.g., rect-v1/6-30° borophane, hex-v1/6-30° borophane, square-borophane, and honeycomb-v1/5 borophane), thus showing that diverse hydrogen bonding motifs on borophene impart ambient stability. As revealed in panel C of
Moreover, similar to the case of hydrogenated graphene, the hydrogenation of borophene is reversible upon thermal annealing. Specifically, borophane samples can be recovered to pristine borophene without apparent degradation after annealing the sample to ˜300° C. (
Synthesis of Borophane Polymorphs:
The synthesis of borophane polymorphs was achieved by a two-step procedure of boron deposition and subsequent hydrogenation in an ultrahigh vacuum (UHV) chamber (base pressure ˜1.0×10−10 mbar). Boron deposition was conducted by electron-beam evaporation (Focus EFM 3) of a solid boron rod (ESPI metals, 99.9999% purity). Borophene was grown on about 600 nm thick Ag(111) on mica substrates (Princeton Scientific Corp.), which were cleaned via repeated cycles of Ar+ sputtering followed by annealing at 550° C. The substrates were held at 400-550° C. during boron deposition, and a flux of 10-15 nA was maintained for 20-30 min for growth of sub-monolayer borophene. Subsequent hydrogenation was conducted by exposing the as-grown borophene sample to atomic hydrogen. The atomic hydrogen was generated by dissociation of molecular hydrogen with a hot tungsten filament held at 1600° C. During hydrogenation, the sample was maintained at room temperature and directly faced the tungsten filament with a distance of ˜10 cm. The chamber pressure during hydrogenation was maintained at 1.0-5.0×10−6 mbar for 1-20 min.
Scanning Tunneling Microscopy and Spectroscopy:
Scanning tunneling microscopy and spectroscopy (STM/STS) were performed in a Scienta Omicron LT system (base pressure ˜5.0×10−11 mbar) at ˜4 K using electrochemically etched Pt/Ir tips. Tip functionalization with CO molecules was obtained by backfilling the STM chamber with 2.0×10−7 mbar of CO for 30 s, with the STM shroud open to allow adsorption of CO onto the cryogenic sample surface. The adsorbed CO molecules were picked up by the STM tip at low sample bias <10 mV. STS measurements were performed using a lock-in amplifier (Signal Recovery 7270) with an amplitude of 3 mVRMS and a modulation frequency of 822 Hz. For field emission resonance (FER) spectroscopy, dI/dV spectra were recorded in constant current mode, which enables a relatively wide voltage window, resulting in the tip continuously retracting from the sample surface during the measurements. For inelastic electron tunneling spectroscopy (LETS), d2I/dV2 spectra were taken with an amplitude of 7 mVRMS and a modulation frequency of 522 Hz. The |dlnI/dz| measurements were performed with a lock-in amplifier (SRS SR850), where an AC output (863 Hz) voltage was added to the z scanner piezo driving signal, causing a 0.3 ÅRMS tip position oscillation. Gwyddion software was used for image processing.
Ex Situ Characterization:
X-ray photoelectron spectroscopy (XPS) measurements were acquired with a Thermo Scientific ESCALAB 250Xi system with a monochromated Al Kα X-ray source. The energy resolution was 0.1 eV with a pass energy of 50 eV. All boron spectra were recorded with an average of 100 scans with a 50 ms dwell time. Avantage (Thermo Scientific) software was used for the calibration and fitting of the XPS spectra. Atomic force microscopy (AFM) was performed in ambient conditions with an Asylum Cypher AFM by Oxford Instruments. The AFM images were obtained in tapping mode (NCHR tips, ˜320 kHz, Nanoworld). The relative humidity was between 20% and 50% throughout the study.
Density Functional Theory Calculations:
Density functional theory (DFT) calculations were performed using the plane-wave code Vienna Ab-initio Simulation Package (VASP) with the projector augmented wave (PAW) method. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation was used for the exchange-correlation functional. To study the effect of interlayer dispersion forces in the stability of the calculated structures, the dispersion-corrected vdW-optB88 exchange-correlation functional was applied. A cutoff energy of 500 eV and an auto-generated Gamma-centered k-point mesh with 50 k-points per Å−1 were used. The total energy was converged to less than 1 meV with respect to the energy cutoff and k-point sampling. To account for the odd number of valence electrons in the simulation cell, the calculations were performed using spin polarization. The numbers of valence electrons in the pseudopotentials for silver, boron, and hydrogen are 11, 3, and 1, respectively. A minimum vacuum spacing of 18 Å was used for all calculations between the periodic images of the slabs.
The simulated STM images were calculated using the python package Ingrained, which performs computer-vision-based image similarity to match the simulated and experimental STM images. The projected density of states (pDOS) of all structures were calculated using the tetrahedron method with Blöchl corrections. The finite-difference approach was used to calculate the vibrational modes of the proposed rect-v1/6-30° borophane structures, which were initially relaxed with an energy convergence criterion of 10−8 eV. The dynamical matrix for vibrational modes was calculated by allowing four ionic displacements with a step size of 0.01 Å in each direction for every boron and hydrogen atom in the cell. Local work functions for all structures were calculated from the planar-averaged electrostatic potential profiles, with dipole corrections to account for the dipole moments between the top and bottom surfaces of the slabs.
The silver substrate included seven atomic layers for structure relaxation, pDOS, and work function calculations, and five layers for simulated STM, vibrational modes, and Bader charge analysis. The bottom four layers of the silver substrate were frozen to resemble the bulk silver lattice for all calculations.
Selective Adsorption of Hydrogen on Borophene:
During the initial hydrogenation of borophene (i.e., low dosing condition with 1.0×10−7 mbar chamber pressure), a preference for H adsorption on certain borophene phases is found.
STM Imaging Hex-v1/6 and Rect-v1/6-30° Borophane Structures with Bare and Functionalized Probes:
Hex-v1/6 borophane was characterized as a disordered structure with a bare STM tip, as confirmed by the fast Fourier transform (FFT) pattern (panels A-B of
Rect-v1/6-30° borophane was imaged as a parallel striped pattern with 0.50±0.02 nm intervals with a bare STM tip, as shown in panel A of
Predominant Rect-v1/6-30° Borophane Phase:
Although coexisting with some hexagonal H patterns, the rectangular H pattern is the predominant phase with the highest coverage on v1/6-30° borophane as observed in
Hydrogen Adsorption Energy Landscapes on v1/6-30° Borophene:
Adsorption energies of H atoms on v1/6-30° borophene were calculated by systematically placing H atoms over an 11×6 grid on the surface of the 5 Å×3 Å unit cell while only allowing H atoms to relax in the z-direction. Panel A of
Stable Alignment of Rect-v1/6-30° Borophane on Ag(111):
The stable alignment of v1/6-30° borophene on the Ag(111) substrate was determined by calculating the total energy when sliding the 2D borophene layer on a 6×4 grid on the Ag(111) surface used for the H adsorption calculations. Panels A-C of
To further compare the translated structures with the experimental results, the simulated STM images of t-rect-1Htop, t-rect-1Hbridge, and t-rect-2H are shown in
The projected density of states (pDOS) of the translated borophane structure (
Local Work Function of Rect-v1/6-30° Borophane:
When the Fermi level of the tip exceeds the sample vacuum level, new current channels associated with field emission resonances (FERs) begin to appear, as schematically shown in panel A of
Following equation (1), the LWF can be estimated by plotting eVn−n2/3 (n≥3) (panel B of
The apparent barrier height (ϕap) can be extracted from the exponential dependence of the tunneling current (I) on the tip-sample separation (z):
From equation (2), ϕap is directly related to |d ln I/dz|. To better visualize the variations in the LWF between borophane polymorphs, |d ln I/dz| maps were acquired simultaneously with constant-current STM imaging (
Local Potential Distribution of the Rect-1Htop Structure:
The side view of the local potential distribution of the rect-1Htop structure along the plane of H atoms is shown in
Bader Analysis of Rect-v1/6-30° Borophane:
Bader analysis was performed on rect-v1/6-30° borophane structures to investigate the charge transfer between the species.
Thermal Stability of Borophane:
In order to test thermal stability, borophane samples were annealed in UHV. After annealing at 200° C. for 1 hour, hydrogen desorption was observed from borophane, as shown in panels C-D of
In summary, a diverse set of borophane polymorphs have been synthesized by exposing borophene to atomic hydrogen in UHV. Among these borophane polymorphs, rect-v1/6-30° borophane showed the most highly ordered domains, thus facilitating further interrogation of its atomic and electronic structure. Through atomically resolved STM, STS, LETS, and DFT calculations, rect-v1/6-30° borophane is found to possess the rect-2H structure, which includes a combination of 2c2e B—H and 3c2e B—H—B bonds. Furthermore, FER spectroscopy, apparent barrier height imaging, and DFT revealed that hydrogenation lowers the LWF of v1/6-30° borophene. FER spectroscopy also showed a splitting of the first FER peak of rect-v1/6-30° borophane, which is consistent with the surface potential corrugation resulting from the combination of 2c2e B—H and 3c2e B—H—B bonds. Chemically, hydrogenation acts as a robust passivation scheme for borophene with multiple borophane polymorphs suppressing oxidation in ambient conditions for timescales on the order of days. Since this hydrogenation can be reversibly removed through thermal annealing, borophane can be returned to pristine borophene following ambient processing, thus presenting significant flexibility in borophene sample preparation. Overall, this work reveals the richness of borophane chemistry, which is likely to inspire and motivate future exploration of covalently modified borophene polymorphs.
The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the invention pertains without departing from its spirit and scope. Accordingly, the scope of the invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.
Some references, which may include patents, patent applications, and various publications, are cited and discussed in the description of this invention. The citation and/or discussion of such references is provided merely to clarify the description of the invention and is not an admission that any such reference is “prior art” to the invention described herein. All references cited and discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference.
This application claims priority to and the benefit of U.S. Provisional Application No. 63/139,382, filed Jan. 20, 2021, which is incorporated herein in its entirety by reference.
This invention was made with government support under 1720139 awarded by the National Science Foundation, and N00014-17-1-2993 awarded by the Office of Naval Research. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/012082 | 1/12/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63139382 | Jan 2021 | US |
