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Aplicação de formulação baseada no método dos elementos finitos posicional na análise bidimensional elástica de compósitos particulados; Application of a positional finite element method based formulation on the elastic two-dimensional analysis of particulate composites

Moura, Camila Alexandrino
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 05/05/2015 PT
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A utilização de materiais compósitos tornou-se uma alternativa importante em muitas aplicações dentro de diversas áreas da engenharia, pois seus constituintes podem agregar propriedades mecânicas, térmicas e acústicas ao compósito, garantindo eficiência e baixo custo. Com isso, faz-se necessário um maior conhecimento do comportamento mecânico desses materiais diante das solicitações, principalmente no que diz respeito aos campos de deslocamento, deformações e tensões. O presente trabalho tem por finalidade a análise, em nível macroscópico, de estruturas bidimensionais elásticas constituídas de materiais compósitos particulados, utilizando formulação desenvolvida no contexto do Grupo de Mecânica Computacional (GMEC), do Departamento de Engenharia de Estruturas (SET), da Escola de Engenharia de São Carlos (EESC), da Universidade de São Paulo (USP), no qual se insere a presente pesquisa. A formulação utilizada baseia-se no Método dos Elementos Finitos Posicional (MEFP) e foi desenvolvida em nível mesoscópico por tratar da interação entre matriz e partículas. Tal formulação possibilita a consideração da interação partícula-matriz sem a necessidade de coincidência entre as malhas da matriz e das partículas e sem o aumento do número de graus de liberdade dos problemas...

A nonzero gap two-dimensional carbon allotrope from porous graphene

Brunetto, Gustavo; Santos, Bruno I.; Autreto, Pedro A. S.; Machado, Leonadro D.; Dos Santos, Ricardo P. B.; Galvao, Douglas S.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Conferência ou Objeto de Conferência Formato: 79-84
ENG
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Graphene has been one of the hottest topics in materials science in the last years. Because of its special electronic properties graphene is considered one of the most promising materials for future electronics. However, in its pristine form graphene is a gapless semiconductor, which poses some limitations to its use in some transistor electronics. Many approaches have been tried to create, in a controlled way, a gap in graphene. These approaches have obtained limited successes. Recently, hydrogenated graphene-like structures, the so-called porous graphene, have been synthesized. In this work we show, based on ab initio quantum molecular dynamics calculations, that porous graphene dehydrogenation can lead to a spontaneous formation of a nonzero gap two-dimensional carbon allotrope, called biphenylene carbon (BC). Besides exhibiting an intrinsic nonzero gap value, BC also presents well delocalized frontier orbitals, suggestive of a structure with high electronic mobility. Possible synthetic routes to obtain BC from porous graphene are addressed. © 2012 Materials Research Society.

2D and 3D periodic templates through holographic interference lithography : photonic and phononic crystals and biomimetic microlens arrays; Two-dimensional and three-dimensional periodic templates through holographic interference lithography

Ullal, Chaitanya K. (Chaitanya Kishore)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 133 leaves; 6752048 bytes; 6757595 bytes; application/pdf; application/pdf
ENG
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In this thesis a simple technique for controlling structure via holographic interference lithography was established and implemented. Access to various space groups including such important structures as the level set approximations to the Diamond, the Schwartz P structure, the FCC, and the non centrosymmetric Gyroid structures were demonstrated. The ability to make 3D structures over a large area, with low defect densities and periodicities on the sub/i scale opens a whole range of opportunities including such diverse areas as photonic crystals, phononic crystals, drug delivery, microtrusses, tissue scaffolds, microfluidics and colloidal crystallization. A correlation between structure and photonic band gap properties was established by systematically exploring the 11 FCC space groups. This resulted in a technique to search for photonic band gap structures. It was found that a fundamental connectivity caused by simple Fourier elements tended to support gaps. 2-3, 5-6 and 8-9 gaps were opened in the f.c.c lattices. The F-RD and 216 structures were newly shown to have complete band gaps. Two of the three previously established champion photonic crystal structures, viz. the Diamond and the Gyroid presented practical fabrication challenges...

Lattice deformations and spin-orbit effects in two dimensional materials

Ochoa de Eguileor Romillo, Héctor
Fonte: Universidade Autônoma de Madrid Publicador: Universidade Autônoma de Madrid
Tipo: Tese de Doutorado
SPA
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Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 18-09-2014; This thesis deals with the interplay between structural and electronic properties of two-dimensional materials such as graphene, and the novel and very interesting phenomena, both from the point of view of fundamental Physics and potential applications, which emerge when lattice distortions such as strains or superlattice modulations are combined with the dynamics of the electrons confined in two spatial dimensions. The main microscopic ingredient which is behind all these phenomena is the spin-orbit interaction. On the one hand, we analyze in detail how the spin-orbit interaction modifies the electronic structure of these materials, and on the other, how structural changes affect the spin-orbit interaction suffered by the electrons of the solid, then modifying its electronic response in a very peculiar manner due to the entanglement of the spin and orbital degrees of freedom. The contents of the thesis are divided in three blocks. The first part is devoted to study the effect of out-of-plane (flexural) vibration modes on the electronic properties of graphene. We examine in detail the influence of the electron-phonon coupling on the mobilities of suspended graphene samples...

A Bond-order Theory on the Phonon Scattering by Vacancies in Two-dimensional Materials

Xie, Guofeng; Shen, Yulu; Wei, Xiaolin; Yang, Liwen; Xiao, Huaping; Zhong, Jianxin; Zhang, Gang
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
Publicado em 28/05/2014 EN
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We theoretically investigate the phonon scattering by vacancies, including the impacts of missing mass and linkages () and the variation of the force constant of bonds associated with vacancies () by the bond-order-length-strength correlation mechanism. We find that in bulk crystals, the phonon scattering rate due to change of force constant is about three orders of magnitude lower than that due to missing mass and linkages . In contrast to the negligible in bulk materials, in two-dimensional materials can be 3–10 folds larger than . Incorporating this phonon scattering mechanism to the Boltzmann transport equation derives that the thermal conductivity of vacancy defective graphene is severely reduced even for very low vacancy density. High-frequency phonon contribution to thermal conductivity reduces substantially. Our findings are helpful not only to understand the severe suppression of thermal conductivity by vacancies, but also to manipulate thermal conductivity in two-dimensional materials by phononic engineering.

Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping

Castellanos-Gomez, Andres; Buscema, Michele; Molenaar, Rianda; Singh, Vibhor; Janssen, Laurens; van der Zant, Herre S. J.; Steele, Gary A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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75.81%
Deterministic transfer of two-dimensional crystals constitutes a crucial step towards the fabrication of heterostructures based on artificial stacking of two-dimensional materials. Moreover, control on the positioning of two-dimensional crystals facilitates their integration in complex devices, which enables the exploration of novel applications and the discovery of new phenomena in these materials. Up to date, deterministic transfer methods rely on the use of sacrificial polymer layers and wet chemistry to some extent. Here, we develop an all-dry transfer method that relies on viscoelastic stamps and does not employ any wet chemistry step. This is found very advantageous to freely suspend these materials as there are no capillary forces involved in the process. Moreover, the whole fabrication process is quick, efficient, clean, and it can be performed with high yield.; Comment: 34 pages, including detailed supporting information

Gas-like adhesion of two-dimensional materials onto solid surfaces

Guo, Zhengrong; Chang, Tienchong; Guo, Xingming; Gao, Huajian
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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65.75%
The adhesion of two-dimensional (2D) materials to other surfaces is so far believed to be a solid-solid mechanical contact. Here, we conduct both atomistic simulations and theoretical modeling to show that there exists a reversible conversion of energy between thermal and mechanical work in the attachment/detachment of 2D materials on/off a surface, indicating that 2D materials adhesion is fundamentally like gas adsorption rather than solid adhesion. We reveal that the underlying mechanism of this intriguing gas-like adhesion for 2D materials is the entropy difference between their freestanding and adhered states. Both the theoretical model and atomistic simulations predict that adhesion induced entropy difference increases with increasing adhesion energy and decreasing equilibrium binding distance. The present findings provide a fundamental guidance toward understanding the adhesion of 2D materials, which is important for designing 2D materials based devices and may have general implications for nanoscale efficient energy conversion.; Comment: 17 pages, 7 figures

Inorganic Graphenylene: A Porous Two-Dimensional Material With Tunable Band Gap

Perim, Eric; Paupitz, Ricardo; Atreto, Pedro Alves da Silva; Galvão, Douglas
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/09/2014
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By means of ab initio calculations we investigate the possibility of existence of a boron nitride (BN) porous two-dimensional nanosheet which is geometrically similar to the carbon allotrope known as biphenylene carbon. The proposed structure, which we called Inorganic Graphenylene (IGP), is formed spontaneously after selective dehydrogenation of the porous Boron Nitride (BN) structure proposed by Ding et al. We study the structural and electronic properties of both porous BN and IGP and it is shown that, by selective substitution of B and N atoms with carbon atoms in these structures, the band gap can be significantly reduced, changing their behavior from insulators to semiconductors, thus opening the possibility of band gap engineering for this class of two-dimensional materials.

Screening for Two dimensional MX$_2$ semiconductors with possible high room temperature mobility

Huang, Zhishuo; Zhang, Wenxu; Zhang, Wanli; Li, Yanrong
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/05/2015
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We calculated the electron mobility of 14 two dimensional semiconductors with composition of MX$_2$, where M (= Mo, W, Sn, Hf, Zr and Pt) is the transition metal, and X is S, Se and Te. We treated the scattering matrix by deformation potential approximation. Long wave longitudinal acoustical and optical phonon scatterings are included. Piezoelectric scattering in the compounds without inversion symmetry is also taken into account. We found that out of the 14 compounds, WSe$_2$, PtS$_2$ and PtSe$_2$, are promising regarding to the possible high electron mobility and finite band gap. The phonon limited mobility in PtSe$_2$ reaches about 3000 cm$^2$V$^{-1}$s$^{-1}$ at room temperature which is the highest among the compounds. The bandgap under the local density approximation is 1.25 eV. Our results can be a guide for experiments to search for better two-dimensional materials for future semiconductor devices.

Two-dimensional octagon-structure monolayer of nitrogen group elements and the related nano-structures

Zhang, Yu; Lee, Jason; Wang, Wei-Liang; Yao, Dao-Xin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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In the purpose of expanding the family of two-dimensional materials, we predict the existence of two-dimensional octa-structure of nitrogen group elements that are composed of squares and octagons in first-principle method based on density functional theory (DFT). From our calculations, electronic structures of all monolayers show that they are semiconductors with indirect (N, P, Bi) and direct (As, Sb) band gaps (0.57-2.61eV). Nano-ribbons of three different unpassivated edges and their band structures are also investigated. Because of the reconstruction on the edges and dangling bonds, there exist ferromagnetic edge states in P, As, Sb nano-ribbons with different edges, and a Dirac point near {\pi} is found in the band structure of one specific N nano-ribbon. These structures may be useful in future applications, such as semiconductor devices, spintronics, hydrogen storage and quantum computation.; Comment: 8 pages, 5 figures, 2 tables

Discrete differential geometry and the properties of conformal two-dimensional materials

Barraza-Lopez, Salvador
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.85%
Two-dimensional materials were first isolated no longer than ten years ago, and a comprehensive understanding of their properties under non-planar shapes is still being developed. Strictly speaking, the theoretical study of the properties of graphene and other two-dimensional materials is the most complete for planar structures and for structures with small deformations from planarity. The opposite limit of large deformations is yet to be studied comprehensively but that limit is extremely relevant because it determines material properties near the point of failure. We are exploring uses for discrete differential geometry within the context of graphene and other two-dimensional materials, and these concepts appear promising in linking materials properties to shape regardless of how large a given material deformation is. A brief account of additional contributions arising from our group to two-dimensional materials that include graphene, stanene and phosphorene is provided towards the end of this manuscript.; Comment: Submitted on December 30, 2014 as an invited contribution to an upcoming issue on Advances in Graphene Science and Engineering. Editors: Jeanie Lau (UC-Riverside), Roland Kawakami (Ohio State) and Arthur Epstein (Ohio State). Accepted version of the manuscript...

Emerging Two-dimensional Materials: graphene and its other structural analogues

Mukhopadhyay, Gautam; Behera, Harihar
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 03/06/2013
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The study of graphene, since its discovery around 2004, is possibly the largest and fastest growing field of research in material science, because of its exotic mechanical, thermal, electronic, optical and chemical properties. The studies of graphene have also led to further research in exploring the field of two dimensional (2D) systems in general. For instance, a number of other 2D crystals (not based on carbon, e.g., boronitrene, silicone, graphane, etc.) have been synthesized or predicted theoretically in recent years. Further, theoretical studies have predicted the possibility of other 2D hexagonal crystals of Ge, SiC, GeC, AlN, GaN, etc. The properties of these 2D materials are very different from their bulk. We shall present the general exotic properties of graphene like 2D systems followed by our computational results on the structural and electronic properties of some of them.; Comment: Invited talk given in "Workshop on New and Nano Materials (WNNM)-2012, 20th-21st January 2012", Institute of Materials Science, Planetarium Building, Bhubaneswar-751013, Odisha, India. 12 pages including the cover page; 8 figures

Harnessing quadratic optical response of two-dimensional materials through active microcavities

Ciattoni, Alessandro; Rizza, Carlo
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 26/06/2014
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65.79%
We propose a method for efficiently harnessing the quadratic optical response of two-dimensional graphene-like materials by theoretically investigating second harmonic generation from a current biased sheet placed within a planar active microcavity. We show that, by tuning the cavity to resonate at the second harmonic frequency, a highly efficient frequency doubling process is achieved (several order of magnitude more efficient than the free-standing sheet). The efficiency of the process is not due to phase-matching, which is forbidden by the localization of the nonlinear quadratic response on the two-dimensional atomic layered material, but it stems from the interplay between the two-dimensional planar geometry of the nonlinear medium and the field oscillation within the active cavity near its threshold. The suggested method can easily be extended to different waves interactions and nonlinearities and therefore it can represent a basic tool for efficiently exploiting nonlinear optical properties of two-dimensional materials.; Comment: 5 pages, 4 figures

Electron counting and a large family of two-dimensional semiconductors

Miao, Mao-sheng; Botana, Jorge; Liu, Jingyao; Yan, Wen
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/02/2015
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Comparing with the conventional semiconductors, the choice of the two dimensional semiconductor (2DSC) materials is very limited. Based on proper electron counting, we propose a large family of 2DSCs, all adopting the same structure and consisting of only main group elements. Using advanced density functional calculations, we demonstrate the attainability of these materials, and show that they cover a large range of lattice constants, band gaps and band edge states, therefore are good candidate materials for heterojunctions. This family of two dimensional materials may pave a way toward fabrication of 2DSC devices at the same thriving level as 3D semiconductors.

Substrate-free layer-number identification of two-dimensional materials: A case of Mo$_{0.5}$W$_{0.5}$S$_2$ alloy

Qiao, Xiao-Fen; Li, Xiao-Li; Zhang, Xin; Shi, Wei; Wu, Jiang-Bin; Chen, Tao; Tan, Ping-Heng
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/05/2015
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Any of two or more two-dimensional (2D) materials with similar properties can be alloyed into a new layered material, namely, 2D alloy. Individual monolayer in 2D alloys are kept together by Van der Waals interactions. The property of multilayer alloys is a function of their layer number. Here, we studied the shear (C) and layer-breathing (LB) modes of Mo$_{0.5}$W$_{0.5}$S$_2$ alloy flakes and their link to the layer number of alloy flakes. The study reveals that the disorder effect is absent in the C and LB modes of 2D alloys, and the monatomic chain model can be used to estimate the frequencies of the C and LB modes. We demonstrated how to use the C and LB mode frequency to identify the layer number of alloy flakes deposited on different substrates. This technique is independent of the substrate, stoichiometry, monolayer thickness and complex refractive index of 2D materials, offering a robust and substrate-free approach for layer-number identification of 2D materials.; Comment: 5 pages, 4 figures

Piezoelectricity in Two-Dimensional Group III Monochalcogenides

Li, Wenbin; Li, Ju
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 25/03/2015
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We find that several layer-phase group-III monochalcogenides, including GaS, GaSe and InSe, are piezoelectric in the monolayer form. First-principles calculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe and InSe are on the same order of magnitude as the earlier discovered two-dimensional piezoelectric materials, such as BN and MoS2 monolayers. Our study expands the family of two dimensional piezoelectric materials, suggesting that strong piezoelectric response can occur in a wide range of two dimensional materials with broken inversion symmetry. The co-existence of piezoelectricity and superior photo-sensitivity in these two-dimensional semiconductors enables the integration of electromechanical and optical sensors on the same material platform.

Phonon transport and thermal conductivity in two-dimensional materials

Gu, Xiaokun; Yang, Ronggui
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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75.92%
Two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have attracted increased interest due to their potential applications in electronics and optoelectronics. Thermal transport in two-dimensional materials could be quite different from three-dimensional bulk materials. This article reviews the progress on experimental measurements and theoretical modeling of phonon transport and thermal conductivity in two-dimensional materials. We focus our review on a few typical two-dimensional materials, including graphene, boron nitride, silicene, transition metal dichalcogenides, and black phosphorus. The effects of different physical factors, such as sample size, strain and defects, on thermal transport in Two-dimensional materials are summarized. We also discuss the environmental effect on the thermal transport of two-dimensional materials, such as substrate and when two-dimensional materials are presented in heterostructures and intercalated with inorganic components or organic molecules.; Comment: 67 pages, 18 figures. Submitted to Annual Review of Heat Transfer

Two-Dimensional Van der Waals Epitaxy Kinetics in a Three-Dimensional Perovskite Halide

Wang, Yiping; Shi, Yunfeng; Xin, Guoqing; Lian, Jie; Shi, Jian
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/09/2015
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The exploration of emerging materials physics and prospective applications of two-dimensional materials greatly relies on the growth control of their thickness, phases, morphologies and film-substrate interactions. Though substantial progresses have been made for the development of two-dimensional films from conventional layered bulky materials, particular challenges remain on obtaining ultrathin, single crystalline, dislocation-free films from intrinsically non-Van der Waals-type three-dimensional materials. In this report, with the successful demonstration of single crystalline ultrathin large scale perovskite halide material, we reveal and identify the favorable role of weak Van der Waals film-substrate interaction on the nucleation and growth of the two-dimensional morphology out of non-layered materials compared to conventional epitaxy. We also show how the bonding nature of the three-dimensional material itself affects the kinetic energy landscape of ultrathin films growth. By studying the formation of fractal perovskites assisted with Monte Carlo simulations, we demonstrate that the competition between the Van der Waals diffusion and surface free energy of the perovskite leads to film thickening, suggesting extra strategies such as surface passivation may be needed for the growth of monolayer and a few layers films.; Comment: Crystal Growth & Design 2015

Two-Dimensional Honeycomb Monolayer of Nitrogen Group Elements and the Related Nano-Structure: A First-Principle Study

Lee, Jason; Tian, Wen-Chuan; Wang, Wei-Liang; Yao, Dao-Xin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.81%
Because of its novel physical properties, two-dimensional materials have attracted great attention. From first-principle calculations and vibration frequenceis analysis, we predict a new family of two-dimensional materials based on the idea of octet stability: honeycomb lattices of pnictogens (N, P, As, Sb, Bi). The buckled structures of materials come from the sp3 hybridization. These materials have indirect band gap ranging from 0.43eV to 3.7eV. From the analysis of projected density of states, we argue that the s and p orbitals together are sufficient to describe the electronic structure under tight-binding model, and the tight-binding parameters are obtained by fitting the band structures to first-principle results. Surprisingly large on-site spin-orbit coupling is found for all the pnictogen lattices except nitrogen. Investigation on the electronic structures of both zigzag and armchair nanoribbons reveals the possible existence of spin-polarized ferromagnetic edge states in some cases, which are rare in one-dimensional systems. These edge states and magnetism may exist under the condition of high vaccum and low temperature. This new family of materials would have promising applications in electronics, optics, sensors, and solar cells.; Comment: 22 pages...

Nature of the quantum metal in a two-dimensional crystalline superconductor

Tsen, Adam; Hunt, B.; Kim, Y. D.; Yuan, Z. J.; Jia, S.; Cava, R. J.; Hone, J.; Kim, Philip; Dean, C. R.; Pasupathy, A. N.
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
EN_US
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Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram, whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe2 in an inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power-law scaling with field in this phase, which is consistent with the Bose-metal model where metallic behaviour arises from strong phase fluctuations caused by the magnetic field.