Crystals

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9 pages, 4420 KiB

Open AccessCommunication

Ab-Initio Calculation of the Electrical Conductivity, Optical Absorption, and Reflectivity of the 2D Materials SnC and NbC

by Nadxiieli Delgado, Osiris Salas, Erick Garcés and Luis Fernando Magaña

Crystals 2023, 13(4), 682; https://doi.org/10.3390/cryst13040682 - 16 Apr 2023

Cited by 1 | Viewed by 2468

Abstract

Using density functional theory (DFT), we performed first-principles calculations of the electrical conductivity, optical absorption, and reflectivity for the 2D carbides SnC and NbC. We calculated the electronic energy band structure of the materials. We performed the calculations without considering the spin–orbit coupling [...] Read more.

Using density functional theory (DFT), we performed first-principles calculations of the electrical conductivity, optical absorption, and reflectivity for the 2D carbides SnC and NbC. We calculated the electronic energy band structure of the materials. We performed the calculations without considering the spin–orbit coupling (SOC) term and including it. We determined that 2D SnC is a semiconductor material and 2D NbC is a conductor. We compared the optical absorption and reflectivity with those of graphene. We found that the 2D SnC and graphene optical absorptions in the infrared region are similar and small; the corresponding values for 2D NbC are approximately ten times larger. In the visible range, the absorption values for 2D SnC and 2D NbC are of the same magnitude and much more significant than graphene. We found that the 2D NbC optical absorption for the ultraviolet region was close to zero. Graphene and 2D SnC have similar maximum values for absorption but at different energies. We determined that graphene reflectivity is larger but similar to that of 2D NbC, and that the 2D SnC reflectivity is near zero. Finally, the 2D NbC electrical conductivity value was about ten times larger than the corresponding value for 2D SnC. As expected, when there was a change of dimensionality, the related 3D materials showed a vastly different value for the electrical conductivity. The 2D materials showed conductivities significantly smaller than those of 3D materials in both cases. The results we obtained for 2D SnC and 2D NbC when we included the SOC term showed that the electrical conductivity for 2D SnC increased by 13.18% and 2D NbC by 18.16%. The optical properties changed, particularly the location of the peaks in the optical absorption and reflectivity. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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12 pages, 3926 KiB

Open AccessArticle

Aluminum Phosphide van der Waals Bilayers with Tunable Optoelectronic Properties under Biaxial Strain

by Caixia Mao, Hao Ni, Libing Qian, Yonghong Hu and Haiming Huang

Crystals 2023, 13(4), 597; https://doi.org/10.3390/cryst13040597 - 1 Apr 2023

Cited by 1 | Viewed by 1308

Abstract

The electronic and optical properties of three types of aluminum phosphide bilayers are examined using density functional theory. The results indicate that they all possess proper direct gaps, which exhibit a rich variety of behaviors depending on the strain. The band gaps of [...] Read more.

The electronic and optical properties of three types of aluminum phosphide bilayers are examined using density functional theory. The results indicate that they all possess proper direct gaps, which exhibit a rich variety of behaviors depending on the strain. The band gaps of these aluminum phosphide bilayers could be easily tuned in the energy range from 0 eV to 1.9 eV under a wide range of biaxial strain. Additionally, band gap transitions between direct and indirect types are found when the external strain applied on them is changed from −12% to 12%. In addition, it was found that these AlP bilayers show strong light-harvesting ability for the ultraviolet light range of the solar spectrum (400–100 nm). The results obtained here indicate that these aluminum phosphide bilayers may have significant potential applications in future nanoelectric fields. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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14 pages, 4429 KiB

Open AccessArticle

Study on Texture Formation of Sb₂Te Thin Films for Phase Change Memory Applications

by Lei Kang and Leng Chen

Crystals 2023, 13(3), 377; https://doi.org/10.3390/cryst13030377 - 22 Feb 2023

Cited by 2 | Viewed by 1359

Abstract

We investigated the texture formation of Sb₂Te thin films for phase change memory applications. The Sb₂Te thin films with different thicknesses were deposited on Si (100) wafers by the magnetron sputtering method. As-deposited Sb₂Te thin films were [...] Read more.

We investigated the texture formation of Sb₂Te thin films for phase change memory applications. The Sb₂Te thin films with different thicknesses were deposited on Si (100) wafers by the magnetron sputtering method. As-deposited Sb₂Te thin films were annealed at various temperatures and times. The texture characterization was performed by using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Experimental results show that the annealed Sb₂Te thin films exhibit the

\{11 \bar{2} 0\}

and

\{10 \bar{1} 0\}

prismatic texture. The formation of prismatic texture is induced by the lattice strain, surface energy, and coarse grains, in which the lattice strain is the essential origin of prismatic preference. Electronic transport properties of Sb₂Te thin films were monitored by a physical property measurement system (PPMS). It was found that the formation of prismatic texture promotes the increase of carrier mobility. The stability of the film–substrate interface was also assessed by calculating mismatch. The prismatic-preferred Sb₂Te thin films exhibit higher mismatch with a silicon wafer, reducing the interface stability. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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15 pages, 8290 KiB

Open AccessArticle

Predicting the Level of Background Current Noise in Graphene Biosensor through a Non-Covalent Functionalization Process

by Chao-yi Zhu, Zi-hong Lin, Da-yong Zhang, Jing-yuan Shi, Song-ang Peng and Zhi Jin

Crystals 2023, 13(2), 359; https://doi.org/10.3390/cryst13020359 - 19 Feb 2023

Cited by 1 | Viewed by 2034

Abstract

The rapid worldwide spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has created a series of problems. Detection platforms based on graphene field-effect transistors (GFETs) have been proposed to achieve a rapid diagnosis of SARS-CoV-2 antigen or antibody. For GFET-based biosensors, [...] Read more.

The rapid worldwide spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has created a series of problems. Detection platforms based on graphene field-effect transistors (GFETs) have been proposed to achieve a rapid diagnosis of SARS-CoV-2 antigen or antibody. For GFET-based biosensors, the graphene surface usually needs to be functionalized to immobilize the bioreceptor and the non-covalent approach is preferred for functionalization because it is believed not to significantly alter the electronic properties of graphene. However, in this work, the non-covalent functionalization introduced by 1-pyrenebutyric acid N-hydroxysuccinimide ester (PBASE) was determined to lead to different changes in electrical properties in graphene samples with different defect densities. The fabricated graphene biosensor can successfully detect SARS-CoV-2 antigen with a concentration as low as 0.91 pg/mL. Further, by careful comparison, we determined that, for GFET fabricated on graphene with a higher defect density, the current variation caused by PBASE modification is greater and the background current noise in the subsequent antigen detection is also larger. Based on this relationship, we can predict the background current noise of the biosensors by evaluating the current change induced by the modification and screen the devices at an early stage of graphene biosensor fabrication for process optimization. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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11 pages, 2858 KiB

Open AccessArticle

Structures, Electric Properties and STM Images of GeSe Monolayers Doped by Group IV–VI Atoms: A First-Principles Study

by Hao Ni, Yonghong Hu, Guopeng Zhou, Caixia Mao, Zhiyuan Chen, Qingyong He and Libing Qian

Crystals 2023, 13(2), 284; https://doi.org/10.3390/cryst13020284 - 7 Feb 2023

Cited by 1 | Viewed by 1620

Abstract

Doping is an important method to modulate the physical and chemical properties of two-dimensional materials. By substitutional doping, different group IV–VI atoms are doped in GeSe monolayers to compose the doped models, of which the effects are investigated using first-principles calculations. The results [...] Read more.

Doping is an important method to modulate the physical and chemical properties of two-dimensional materials. By substitutional doping, different group IV–VI atoms are doped in GeSe monolayers to compose the doped models, of which the effects are investigated using first-principles calculations. The results show that local deformations of geometrical structure can be observed around the doping atoms. According to the analysis of the formation energy and the cohesive energy, all the doped models have a strongly bonded network, and GeSe_N possesses the most stable structure. Only the bandgap of Ge_As is direct, while those of other doped models are indirect. Thus, direct and indirect bandgaps are alternative by doping different atoms. The structural and electronic properties, especially for the bond lengths variation around doping atoms, are explained by the charge density difference. Finally, the scanning tunnel microscope images of the doped models are analyzed for further experimental investigations. Our work provides a stimulating account by atom doping which could trigger the developments and applications of new two-dimensional materials for manufacturing microelectronic and optoelectronic devices. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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12 pages, 4615 KiB

Open AccessArticle

Multiple Exceptional Points in APT–Symmetric Cantor Multilayers

by Ming Fang, Yang Wang, Pu Zhang, Haihong Xu and Dong Zhao

Crystals 2023, 13(2), 197; https://doi.org/10.3390/cryst13020197 - 22 Jan 2023

Cited by 2 | Viewed by 1376

Abstract

In this study, we explore the anisotropic reflection of light waves around the exceptional points (EPs) in anti-parity-time−symmetric (APT−symmetric) Cantor dielectric multilayers. This one-dimensional fractal structure governed by the Cantor substitution law is modulated to satisfy APT symmetry. The Cantor multilayers are aperiodic [...] Read more.

In this study, we explore the anisotropic reflection of light waves around the exceptional points (EPs) in anti-parity-time−symmetric (APT−symmetric) Cantor dielectric multilayers. This one-dimensional fractal structure governed by the Cantor substitution law is modulated to satisfy APT symmetry. The Cantor multilayers are aperiodic and support optical fractal resonances. The optical fractal effect combined with APT symmetry can induce multiple exceptional points (EPs) in the parameter space by modulating the loss coefficient of materials and optical frequency. Reflection anisotropy for light waves incident from two opposite directions presents unidirectional suppression and enhancement around EPs. This study can be utilized for multiple wavelengths of photonic suppressors and reflectors. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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11 pages, 2666 KiB

Open AccessArticle

Boron Nanotube Structure Explored by Evolutionary Computations

by Tomasz Tarkowski and Nevill Gonzalez Szwacki

Crystals 2023, 13(1), 19; https://doi.org/10.3390/cryst13010019 - 23 Dec 2022

Cited by 1 | Viewed by 1732

Abstract

In this work, we explore the structure of single-wall boron nanotubes with large diameters (about 21 Å) and a broad range of surface densities of atoms. The computations are done using an evolutionary approach combined with a nearest-neighbors model Hamiltonian. For the most [...] Read more.

In this work, we explore the structure of single-wall boron nanotubes with large diameters (about 21 Å) and a broad range of surface densities of atoms. The computations are done using an evolutionary approach combined with a nearest-neighbors model Hamiltonian. For the most stable nanotubes, the number of 5-coordinated boron atoms is about

63 %

of the total number of atoms forming the nanotubes, whereas about

11 %

are boron vacancies. For hole densities smaller than about 0.22, the boron nanotubes exhibit randomly distributed hexagonal holes and are more stable than a flat stripe structure and a quasi-flat B₃₆ cluster. For larger hole densities (>0.22), the boron nanotubes resemble porous tubular structures with hole sizes that depend on the surface densities of boron atoms. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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14 pages, 5485 KiB

Open AccessArticle

Charge State Effects in Swift-Heavy-Ion-Irradiated Nanomaterials

by Kristina Tomić Luketić, Juraj Hanžek, Catalina G. Mihalcea, Pavo Dubček, Andreja Gajović, Zdravko Siketić, Milko Jakšić, Corneliu Ghica and Marko Karlušić

Crystals 2022, 12(6), 865; https://doi.org/10.3390/cryst12060865 - 19 Jun 2022

Cited by 9 | Viewed by 2437

Abstract

The aim of this experimental work was to investigate the influence of the ion beam charge state on damage production in nanomaterials. To achieve this, we employed Raman spectroscopy, atomic force microscopy, and transmission electron microscopy to investigate nanomaterials irradiated by a 23 [...] Read more.

The aim of this experimental work was to investigate the influence of the ion beam charge state on damage production in nanomaterials. To achieve this, we employed Raman spectroscopy, atomic force microscopy, and transmission electron microscopy to investigate nanomaterials irradiated by a 23 MeV I beam. We found a significant influence of the ion charge state on damage production in monolayer graphene, but found no evidence of this effect in bilayer and trilayer graphene, nor in graphite. Furthermore, we found no evidence of this effect in CaF₂ and SiO₂ nanocrystals irradiated with the same ion beam. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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10 pages, 3373 KiB

Open AccessArticle

Highly Efficient Contact Doping for High-Performance Organic UV-Sensitive Phototransistors

by Bin Li, Yihan Zhang, Yang Liu, Yiwen Ren, Xiaoting Zhu, Lingjie Sun, Xiaotao Zhang, Fangxu Yang, Rongjin Li and Wenping Hu

Crystals 2022, 12(5), 651; https://doi.org/10.3390/cryst12050651 - 2 May 2022

Cited by 6 | Viewed by 3407

Abstract

Organic ultraviolet (UV) phototransistors are promising for diverse applications. However, wide-bandgap organic semiconductors (OSCs) with intense UV absorption tend to exhibit large contact resistance (R_c) because of an energy-level mismatch with metal electrodes. Herein, we discovered that the molecular dopant [...] Read more.

Organic ultraviolet (UV) phototransistors are promising for diverse applications. However, wide-bandgap organic semiconductors (OSCs) with intense UV absorption tend to exhibit large contact resistance (R_c) because of an energy-level mismatch with metal electrodes. Herein, we discovered that the molecular dopant of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) was more efficient than the transition metal oxide dopant of MoO₃ in doping a wide-bandgap OSC, although the former showed smaller electron affinity (EA). By efficient contact doping, a low R_c of 889 Ω·cm and a high mobility of 13.89 cm²V⁻¹s⁻¹ were achieved. As a result, UV-sensitive phototransistors showed high photosensitivity and responsivity. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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9 pages, 1587 KiB

Open AccessArticle

Hot-Pressed Two-Dimensional Amorphous Metals and Their Electronic Properties

by Jieying Liu, Jian Tang, Jiaojiao Zhao, Yanchong Zhao, Cheng Shen, Mengzhou Liao, Shuopei Wang, Jinpeng Tian, Yanbang Chu, Jiawei Li, Zheng Wei, Gen Long, Wei Yang, Rong Yang, Na Li, Dongxia Shi and Guangyu Zhang

Crystals 2022, 12(5), 616; https://doi.org/10.3390/cryst12050616 - 26 Apr 2022

Viewed by 2662

Abstract

As an emerging research field, two-dimensional (2D) metals have been the subject of increasing research efforts in recent years due to their potential applications. However, unlike typical 2D layered materials, such as graphene, which can be exfoliated from their bulk parent compounds, it [...] Read more.

As an emerging research field, two-dimensional (2D) metals have been the subject of increasing research efforts in recent years due to their potential applications. However, unlike typical 2D layered materials, such as graphene, which can be exfoliated from their bulk parent compounds, it is hardly possible to produce 2D metals through exfoliation techniques due to the absence of Van der Waals gaps. Indeed, the lack of effective material preparation methods severely limits the development of this research field. Here, we report a PDMS-assisted hot-pressing method in glovebox to obtain ultraflat nanometer-thick 2D metals/metal oxide amorphous films of various low-melting-point metals and alloys, e.g., gallium (Ga), indium (In), tin (Sn), and Ga_0.87Ag_0.13 alloy. The valence states extracted from X-ray photoelectron spectroscopy (XPS) indicate that the ratios of oxidation to metal in our 2D films vary among metals. The temperature-dependent electronic measurements show that the transport behavior of 2D metal/metal oxide films conform with the 2D Mott’s variable range hopping (VRH) model. Our experiments provide a feasible and effective approach to obtain various 2D metals. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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11 pages, 2803 KiB

Open AccessArticle

Seeding-Layer-Free Deposition of High-k Dielectric on CVD Graphene for Enhanced Gate Control Ability

by Yunpeng Yan, Songang Peng, Zhi Jin, Dayong Zhang and Jingyuan Shi

Crystals 2022, 12(4), 513; https://doi.org/10.3390/cryst12040513 - 7 Apr 2022

Cited by 4 | Viewed by 2178

Abstract

The gate insulator is one of the most crucial factors determining the performance of a graphene field effect transistor (GFET). Good electrostatic control of the conduction channel by gate voltage requires thin gate oxides. Due to the lack of the dangling bond, a [...] Read more.

The gate insulator is one of the most crucial factors determining the performance of a graphene field effect transistor (GFET). Good electrostatic control of the conduction channel by gate voltage requires thin gate oxides. Due to the lack of the dangling bond, a seed layer is usually needed for the gate dielectric film grown by the atomic layer deposition (ALD) process. The seed layer leads to the high-quality deposition of dielectric films, but it may lead to a great increase in the thickness of the final dielectric film. To address this problem, this paper proposes an improved process, where the self-oxidized Al₂O₃ seed layer was removed by etching solutions before atomic layer deposition, and the Al₂O₃ residue would provide nucleation sites on the graphene surface. Benefiting from the decreased thickness of the dielectric film, the transconductance of the GFET using this method as a top-gate dielectric film deposition process shows an average 44.7% increase compared with the GFETs using the standard Al evaporation seed layer methods. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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8 pages, 909 KiB

Open AccessArticle

Transport Properties of the Two-Dimensional Hole Gas for H-Terminated Diamond with an Al₂O₃ Passivation Layer

by Cui Yu, Chuangjie Zhou, Jianchao Guo, Zezhao He, Mengyu Ma, Hongxing Wang, Aimin Bu and Zhihong Feng

Crystals 2022, 12(3), 390; https://doi.org/10.3390/cryst12030390 - 14 Mar 2022

Cited by 2 | Viewed by 2037

Abstract

Diamonds are thought to be excellent candidates of next-generation semiconductor materials for high power and high frequency devices. A two-dimensional hole gas in a hydrogen-terminated diamond shows promising properties for microwave power devices. However, high sheet resistance and low carrier mobility are still [...] Read more.

Diamonds are thought to be excellent candidates of next-generation semiconductor materials for high power and high frequency devices. A two-dimensional hole gas in a hydrogen-terminated diamond shows promising properties for microwave power devices. However, high sheet resistance and low carrier mobility are still limiting factors for the performance improvement of hydrogen-terminated diamond field effect transistors. In this work, the carrier scattering mechanisms of a two-dimensional hole gas in a hydrogen-terminated diamond are studied. Surface roughness scattering and ionic impurity scattering are found to be the dominant scattering sources. Impurity scattering enhancement was found for the samples after a high-temperature Al₂O₃ deposition process. This work gives some insight into the carrier transport of hydrogen-terminated diamonds and should be helpful for the development of diamond field effect transistors. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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13 pages, 7159 KiB

Open AccessArticle

Established Model on Polycrystalline Graphene Oxide and Analysis of Mechanical Characteristic

by Xu Xu, Chaoqi Xiong, Shaoping Mao and Wenjuan Yao

Crystals 2022, 12(3), 382; https://doi.org/10.3390/cryst12030382 - 12 Mar 2022

Cited by 2 | Viewed by 1737

Abstract

It may cause more novel physical effects that the combination with in-plane defects induced by grain boundaries (GBs) and quasi three-dimensional system induced by oxidation functional group. Different from those in blocks, these new physical effects play a significant role in the mechanical [...] Read more.

It may cause more novel physical effects that the combination with in-plane defects induced by grain boundaries (GBs) and quasi three-dimensional system induced by oxidation functional group. Different from those in blocks, these new physical effects play a significant role in the mechanical properties and transport behavior. Based on the configuration design, we investigate the in-plane and out-plane geometric deformation caused by the coupling of GBs and oxygen-containing functional groups and establish a mechanical model for the optimal design of the target spatial structure. The results show that the strain rate remarkably affect the tensile properties of polycrystalline graphene oxide (PGO). Under high oxygen content (R = 50%), with the increasing strain rate, the PGO is much closer to ductile fracture, and the ultimate strain and stress will correspondingly grow. The growth of temperature reduces the ultimate stress of PGO, but the ultimate strain remains constant. When the functional groups are distributed at the edge of the GBs, the overall strength decreases the most, followed by the distribution on the GBs. Meanwhile, the strength of PGO reaches the greatest value when the functional groups are distributed away from the GBs. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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8 pages, 26580 KiB

Open AccessArticle

Electric-Field Induced Doping Polarity Conversion in Top-Gated Transistor Based on Chemical Vapor Deposition of Graphene

by Songang Peng, Jing Zhang, Zhi Jin, Dayong Zhang, Jingyuan Shi and Shuhua Wei

Crystals 2022, 12(2), 184; https://doi.org/10.3390/cryst12020184 - 27 Jan 2022

Cited by 10 | Viewed by 2995

Abstract

The top-gated graphene field effect transistor (GFET) with electric-field induced doping polarity conversion has been demonstrated. The polarity of channel conductance in GFET can be transition from p-type to n-type through altering the gate electric field scanning range. Further analysis indicates that this [...] Read more.

The top-gated graphene field effect transistor (GFET) with electric-field induced doping polarity conversion has been demonstrated. The polarity of channel conductance in GFET can be transition from p-type to n-type through altering the gate electric field scanning range. Further analysis indicates that this complementary doping is attributed to the charge exchange between graphene and interface trap sites. The oxygen vacancies in Al₂O₃filmare are considered to be the origin of the trap sites. The trapping–detrapping process, which may be tuned by the electric field across the metal/oxide/graphene gate stack, could lead to the changing of the intrinsic electric property of graphene. This study promises to produce the complementary p- and n-type GFET for logic applications. Full article

(This article belongs to the Special Issue 2D Crystalline Nanomaterials)

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