Showing posts with label 2d materials. Show all posts
Showing posts with label 2d materials. Show all posts

Tuesday, January 19, 2021

Atomic Layer Deposition of 2D Metal Dichalcogenides for Electronics, Catalysis, Energy Storage, and Beyond

Here is an interesting Review from the leading ALD Laboratory at Helsinki University in Finland on 2D dichalcogenides. We are all looking forward to get to know the ALD:ed Dichalcogenides better in the future in exciting new  devices and daily life. (Thanks for sharing - Dr. King)

Atomic Layer Deposition of 2D Metal Dichalcogenides for Electronics, Catalysis, Energy Storage, and Beyond
Miika Mattinen, Markku Leskelä, Mikko Ritala
First published: 18 January 2021 in Advanced Materials Interfaces


Figure from Google cache (originally in https://doi.org/10.1002/admi.202001677)

Abstract: 2D transition metal dichalcogenides (TMDCs) are among the most exciting materials of today. Their layered crystal structures result in unique and useful electronic, optical, catalytic, and quantum properties. To realize the technological potential of TMDCs, methods depositing uniform films of controlled thickness at low temperatures in a highly controllable, scalable, and repeatable manner are needed. Atomic layer deposition (ALD) is a chemical gas‐phase thin film deposition method capable of meeting these challenges. In this review, the applications evaluated for ALD TMDCs are systematically examined, including electronics and optoelectonics, electrocatalysis and photocatalysis, energy storage, lubrication, plasmonics, solar cells, and photonics. This review focuses on understanding the interplay between ALD precursors and deposition conditions, the resulting film characteristics such as thickness, crystallinity, and morphology, and ultimately device performance. Through rational choice of precursors and conditions, ALD is observed to exhibit potential to meet the varying requirements of widely different applications. Beyond the current state of ALD TMDCs, the future prospects, opportunities, and challenges in different applications are discussed. The authors hope that the review aids in bringing together experts in the fields of ALD, TMDCs, and various applications to eventually realize industrial applications of ALD TMDCs

Thursday, January 7, 2021

How ALD can be used to stack 2D materials on one another at a nanometer scale

TU Eindhoven latest publication to see how the toolbox of ALD can be used to stack various layered 2D materials on one another at a nanometer scale.





Thursday, December 17, 2020

Imec introduces 2D materials in the logic device scaling roadmap

[IEDM 2020 Virtual, Imec Belgium LINK] At the 2020 IEDM conference, imec proposes that 2D semiconductors like tungsten disulfide (WS2) can further extend the logic transistor scaling roadmap. The team laid the groundwork for integrating 2D semiconductors in a 300mm CMOS fab, and worked towards improved device performance. These findings are presented in four IEDM papers, one of which was selected as IEDM highlight.

More details can be found in 4 papers presented at the 2020 IEDM conference:

[1] ‘Introducing 2D-FETs in device scaling roadmap using DTCO’, Z. Ahmed et al.
[2] ‘Wafer-scale integration of double gated WS2-transistors in 300mm Si CMOS fab’, I. Asselberghs et al.
[3] ‘Dual gate synthetic WS2 MOSFETs with 120µS/µm Gm 2.7µF/cm2 capacitance and ambipolar channel’, D. Lin et al.
[4] ‘Sources of variability in scaled MoS2 FETs’, Q. Smets et al. (IEDM highlight paper)

TEM image of a 2D device fabricated with 300mm processes. (Source: Imec)


Monday, December 7, 2020

High-quality HfS2 2D-material by ALD at 100°C

Friday, January 24, 2020

Russian researchers obtain atomically thin molybdenum disulfide (2D) films on large-area substrates by ALD

[Press release: LINK] Researchers from the Moscow Institute of Physics and Technology have managed to grow atomically thin films of molybdenum disulfide spanning up to several tens of square centimeters. It was demonstrated that the material’s structure can be modified by varying the synthesis temperature. The films, which are of interest to electronics and optoelectronics, were obtained at 900-1,000 degrees Celsius. The findings were published in the journal ACS Applied Nano Materials.



An atomic layer deposition reactor from Picosun used for obtaining ultrathin molybdenum oxide films, which were subsequently sulfurized to 2D molybdenum disulfide. Image courtesy of the Atomic Layer Deposition Lab, MIPT

Two-dimensional materials are attracting considerable interest due to their unique properties stemming from their structure and quantum mechanical restrictions. The family of 2D materials includes metals, semimetals, semiconductors, and insulators. Graphene, which is perhaps the most famous 2D material, is a monolayer of carbon atoms. It has the highest charge-carrier mobility recorded to date. However, graphene has no band gap under standard conditions, and that limits its applications.



Wednesday, December 11, 2019

Imec shows excellent performance in ultra-scaled FETs with 2D-material channel

[Press release, imec, LINK] SAN FRANCISCO (USA), December 8, 2019 — At this year’s IEEE International Electron Devices Meeting (Dec 7-11 2019), imec, a world-leading research and innovation hub in nanoelectronics and digital technologies, reports an in-depth study of scaled transistors with MoS2 and demonstrates best device performance to date for such materials. 

TEM pictures showing (a) 3 monolayers MoS2 channel, with contact length 13nm and channel length 29nm Transfer characteristics have improved sub-threshold swing (SS) with thinner HfO2. (www.imec.be)

MoS2 is a 2D material, meaning that it can be grown in stable form with nearly atomic thickness and atomic precision. Imec synthesized the material down to monolayer (0.6nm thickness) and fabricated devices with scaled contact and channel length, as small as 13nm and 30nm respectively. These very scaled dimensions, combined with scaled gate oxide thickness and high K dielectric, have enabled the demonstration of some of the best device performances so far. Most importantly, these transistors enable a comprehensive study of fundamental device properties and calibration of TCAD models. The calibrated TCAD model is used to propose a realistic path for performance improvement. The results presented here confirm the potential of 2D-materials for extreme transistor scaling – benefiting both high-performance logic and memory applications.

Thursday, August 8, 2019

Atomic Layer Deposition of Emerging 2D Semiconductors, HfS2 and ZrS2, for Optoelectronics

Miika Mattinen from Prof. Mikko Ritala's group, University of Helsinki, reports the ALD growth of 2D HfS2 and ZrS2—the potential rivals of the hot favorite 2D semiconductors MoS2 and WSe2. 



Abstract: Semiconducting 2D materials are studied intensively because of their promising performance in diverse applications from electronics to energy storage and catalysis. Recently, HfS2 and ZrS2 have emerged as potential rivals for the commonly studied 2D semiconductors such as MoS2 and WSe2, but their use is hindered by the difficulty of producing continuous films. 

Herein, we report the first atomic layer deposition (ALD) processes for HfS2 and ZrS2 using HfCl4 and ZrCl4 with H2S as the precursors. We demonstrate the deposition of uniform and continuous films on a range of substrates with accurately controlled thicknesses ranging from a few monolayers to tens of nanometers. The use of semiconductor industry-compatible precursors and temperatures (approximately 400 °C) enables facile upscaling of the process. The deposited HfS2 and ZrS2 films are crystalline, smooth, and stoichiometric with oxygen as the main impurity. 


----------------------
By Abhishekkumar Thakur

Sunday, April 1, 2018

ALD yields large crystalline 2D MoS2 thin films

MRS Bulletin reports: Sheets of molybdenum disulfide (MoS2) just a few atoms thick hold promise for high-performance, flexible electronics as well as optical applications. But one obstacle the two-dimensional (2D) material faces is the lack of an efficient method to make it in large quantities. Researchers at Argonne National Laboratory have now demonstrated that the atomic layer deposition method could be used to make uniform, crystalline MoS2 thin films as large as a standard 300 mm silicon wafer. 

Full article : LINK 
JVSTA Journal article : LINK
ALD Process: molybdenum hexafluoride (MoF6) and hydrogen sulfide at 200 °C

Saturday, January 6, 2018

New ALD High-k / 2D MoS2 light-erasable memory suitable for large area manufacturing technology

Phys.Org reports that researchers at the Institute of Microelectronics Chinese Academy of Sciences (IMECAS), and Fudan University have used 2D MoS2 to design a new light-erasable memory.

According to the article in Applied Physics Letter, the memory stack is based on an high-k dielectric stack (Al2O3/HfO2/Al2O3) and an atomically thin MoS2 channel, where he HfO2 act as the charge trapping layer. The holes in the HfO2 charge-trapping layer will tunnel to the MoS2 channel through the 4 nm Al2O3 tunnel layer. 
 
 
Schematic band diagrams of the MoS2/Al2O3/HfO2/Al2O3/Gate structure at (a) flat-band condition, (b) programming operation, and (c) erasing operation. (Supplementary information, Applied Physics Letters. DOI: 10.1063/1.5000552)

"In general, system-on-panel (SOP) describes a new display technology in which both active and passive components are integrated in one panel package, typically on the same glass substrate (sometimes system-on-panel is also named system-on-glass)," coauthor Hao Zhu at Fudan University told Phys.org. "This is different from traditional display technologies such as cathode ray tube (CRT) displays. One major characteristic of SOP is the application of thin-film technology, such as low-temperature poly-silicon (LTPS) thin-film transistor (TFT) arrays on the glass substrate. However, silicon-based thin-film transistors are being replaced by TFTs with new materials with improved properties. The indium gallium zinc oxide (IGZO) or zinc tin oxide (ZTO) thin film mentioned in our paper is also a good example. [Phys.org]

"Currently, we are working on the large-scale integration of such light-erasable 2-D memory devices using programmable light pulses with controllable wavelength and pulse duration," he said. "We are using new material synthesis methods such as atomic layer deposition to grow large-area MoS2 and other 2-D ultra-thin films for circuit-level applications." 
[Phys.org]

The future prospects for large scale manufacturing are there. Except for the MoS2 channel, both Al2O3 and HfO2 are standard ALD processing technologies since more than 10 years in the semiconductor industry and recent developments for flexible OLED Display manufacturing  has made the ALD technology also available for large panel processing and roll to roll technology is just looking for an excuse high volume manufacturing.
 
Article: Long-Fei He et al. "Light-erasable embedded charge-trapping memory based on MoS2 for system-on-panel applications." Applied Physics Letters. DOI: 10.1063/1.5000552

Full story: LINK

Wednesday, September 27, 2017

AIXTRON provides novel deposition system to EPFL for 2D materials research

Leading Swiss university focuses on the development of next-generation semiconductors based on AIXTRON BM system

 
AIXTRON SE (FSE: AIXA), a worldwide leading provider of deposition equipment to the semiconductor industry, today announced that the École Polytechnique Fédérale de Lausanne (EPFL) in Lausanne (Switzerland) has purchased a BM NOVO system. This versatile tool which can produce virtually all variations of 2-dimensional materials (2D) required for emerging optoelectronic applications is dedicated to support the University’s research projects coordinated by Prof. Andras Kis and Prof. Aleksandra Radenovic.

AIXTRON’s BM NOVO system uses a unique combination of plasma-enhanced chemical vapor deposition (PECVD) technology and metal organic chemical vapor deposition (MOCVD) technology to enable the growth of high quality 2D materials such as transition metal dichalcogenides (TMDCs) e.g. molybdenum disulfide (MoS2) or tungsten diselenide (WSe2).

Source: Aixtron LINK

Wednesday, August 30, 2017

Webinar - ALD for 2D materials

Oxford Instruments is running a webinar on ALD for 2D materials & devices on 14 September, 3:30pm (UK time). The webinar will comprise of two talks, with a Q&A session at the end:
  • Atomic Layer Deposition for Graphene devices by Dr Daniel Neumaier, AMO GmbH
  • Atomic Layer Deposition on and of 2D materials by Dr Harm Knoops, Oxford Instruments
If you would like to register please visit www.oxinst.com/ALD2D


Tuesday, August 22, 2017

Woah - Hafnium oxide as gate dielectric scales also in the 2D world

Hafnium oxide high-k dielectrics deposited by atomic layer deposition have been used in DRAM since 2004 (Samsung 90 nm) and 2007 in high performance CMOS logic (Intel 45 nm). Later the DRAM high-k dielectric was replaced by a zirconium oxide based material but for logic hafnium oxide has remained the material of choice for the high-k metal gate stack by toping off the native oxide of silicon with its higher k-value. Hafnium oxide even survived the transition to narrow 3D FinFET devices and is also the main contender for silicon based Nano Wire FETs. However, recent research in alternative 2D channel materials such as graphene, molybdenum disulfide and others has created a totally new situation where hafnium oxide finds it difficult to compete as the material of choice for the gate stack dielectric. 

Until now that is, because just recently some clever researchers at Stanford has presented an new all hafnium channel and dielectric combo using hafnium diselenide and the natural native oxide of that - ta da - hafnium oxide. Apparently the zirconium version is also brought into play but let us see about that...

You can read all about it in this online article published by Stanford, which also leads you to the original scientific references and journal publications.

New ultrathin semiconductor materials exceed some of silicon’s ‘secret’ powers, Stanford engineers find

The next generation of feature-filled and energy-efficient electronics will require computer chips just a few atoms thick. For all its positive attributes, trusty silicon can’t take us to these ultrathin extremes.

Now, electrical engineers at Stanford have identified two semiconductors – hafnium diselenide and zirconium diselenide – that share or even exceed some of silicon’s desirable traits, starting with the fact that all three materials can “rust.”



TEM cross-section of an experimental chip, the bands of black and white reveal alternating layers of hafnium diselenide – an ultrathin semiconductor material – and the hafnium dioxide insulator. (Image credit: Michal Mleczko)

Sunday, July 2, 2017

New process for 2D MoS2 from Oxford Instruments

Scientists at Plasma Technology have developed a new process to deposit 2D MoS2 layers directly on Graphene, creating atomic layer heterostructures. This graphene-semiconductor film is a functional layer with applications in photodetection and sensing.

Source: Oxford Instruments LINK 

MoS2 grown on Grephene (Oxford Instruments)

Tuesday, April 25, 2017

Atomic Layer Deposition on 2D Materials by Incheon National University



Here is my new favorite ALD Research Group - The HBRL Group of Prof. Han-Bo-Ram Lee at Incheon National University in South Korea. Please do visit their web and get inspired. Just recently they published an article on ALD on 2D Materials: 

Atomic Layer Deposition on 2D Materials
Hyun Gu Kim and Han-Bo-Ram Lee*
Department of Materials Science and Engineering, Incheon National University, Incheon 22012, Korea
Chem. Mater., Article ASAP
DOI: 10.1021/acs.chemmater.6b05103
Publication Date (Web): April 25, 2017


Screen dump from The HBRL Group of Prof. Han-Bo-Ram Lee at Incheon National University in South Korea (LINK)

Thursday, September 8, 2016

Atomic layer deposition for two-dimensional materials (ALDfor2D) workshop

The aim of this one-day workshop is to give an overview of current topics in the field of atomic layer deposition (ALD) for the synthesis and integration of 2D Materials such as graphene and the transition metal dichalchalcogenides for nanodevice applications. The workshop is geared towards both scientists who work in the field as well as newcomers and technologists who want to get an overview of the field. The workshop is organized in the context of the COST action "HERALD" (MP1402).

Key facts

Date: October 31, 2016
Location: Eindhoven, The Netherlands
Participation: Free
Registration & more details: http://nanomanufacturing.nl/2DWorkshop/

Invited speakers

  • Prof. Robert Wallace (University of Texas at Dallas, USA)
  • Prof. Kim (Yonsei University, South Korea)
  • Dr. Annelies Delabie (IMEC, Belgium)
  • Dr. Ravi Sundaram (OIPT, UK)
  • More to follow!

The workshop is sponsored by

Thursday, July 21, 2016

Ultratech CNT in Dublin at ALD2016 presenting latest work on Superconductivity, SAMS Area-Selective ALD (ASD), Molybdenum Nitride and product developments

The Platinum Sponsor, supporting over 450 ALD research systems worldwide, will bring its science team to the ALD conference to present latest work and discuss leading edge application developments with delegates. Available at booth #15 and presenting as follows.


Plasma enhanced atomic layer deposition of molybdenum nitride
Adam Bertuch*1, Brent Keller2, Ganesh Sundaram1, Jeffrey Grossman2
1Ultratech - Cambridge NanoTech, USA, 2Department of Material Science and Engineering, Massachusetts Institute of Technology, USA
Tuesday 26 July:  Plasma-enhanced deposition and etching  -  Tuesday 26 July 15:45-17:15
16:30-16:45
Controlling smoothness of thin platinum ALD films
Ritwik Bhatia*1, Ralf Heilmann2, Alexander Bruccoleri3, Brandon Chalifoux2
1Ultratech-Cambridge Nanotech, USA, 2Massachusetts Institute of Technology, USA, 3Izentis LLC, USA
Wednesday 27 JULY:  Noble metals  -  Wednesday 27 July 08:15-10:15
10:00-10:15
Plasma enhanced atomic layer deposition of superconducting NbN films
Mark Sowa*1, Yonas Yemane2, J Provine3, Fritz Prinz4
1Ultratech/CNT, USA, 2Stanford University, Department of Applied Physics, USA, 3Stanford University, Department of Electrical Engineering, USA, 4Stanford University, Department of Mechanical Engineering and Department of Materials Science and Engineering, USA
Tuesday 26 July:  Poster session 2  -  Tuesday 26 July 17:15-19:00


Thursday, February 11, 2016

Oregon State present ALD of 2D alternate channel material MoS2 on 6 inch wafers

Graphene has a big problem - it lacks a bandgap which is needed for many electronic devices and this has led searching of alernate 2D materials. Most focus today is on transition metal dichalcogenides (TMDs). One of the most promising TMDs is molybdenum disulfide (MoS) with a bandgap (∼1.2 eV) for bulk MoS and a direct bandgap (∼1.8 eV) in the mono layer form monolayer.

Some weeks ago it was reported that ALD sales booming for Arradiance GEMStar XT line. Here is an Open Source paper in JVSTA on depositing 2D MoS2 by alternate pulsing of MoCl5 and H2S on 6 inch wafers using an Arradiance GEMStar ALD reactor by School of EECS, Oregon State University and Sharp Lab of America.
 

Installed Fall 2010: Arradiance Gemstar (see press release); 150mm ALD reactor with 3D substrate capability, in-situ quartz crystal microbalance, and 5 precursor source lines (1 gas; 2 vapour draw for liquids; and 2 low vapor pressure sources, heated up to 120C, one with N2 boost)

Here you can find more details on the research and facilities of Prof. John F. Conley`s Novel Materials and Devices Group at Oregon State: http://web.engr.oregonstate.edu/~jconley/facilities.html. In Addition to the Arradiance GEMStar they are operating a Picosun SUNALE R-200 200mm Plasma Enhanced ALD system.

Atomic layer deposition of two dimensional MoS on 150 mm substrates

Arturo Valdivia, Douglas J. Tweet and John F. Conley Jr.
J. Vac. Sci. Technol. A 34, 021515 (2016); http://dx.doi.org/10.1116/1.4941245

Low temperature atomic layer deposition(ALD) of monolayer to few layer MoS uniformly across 150 mm diameter SiO/Si and quartz substrates is demonstrated. Purge separated cycles of MoCl and HS precursors are used at reactor temperatures of up to 475 °C. Raman scattering studies show clearly the in-plane (E1) and out-of-plane (A) modes of MoS. The separation of the E1 and A peaks is a function of the number of ALD cycles, shifting closer together with fewer layers. X-ray photoelectron spectroscopy indicates that stoichiometry is improved by postdeposition annealing in a sulfur ambient. High resolution transmission electron microscopy confirms the atomic spacing of monolayer MoS thin films.

Tuesday, January 19, 2016

Large area Self-Limiting Synthesis of Transition Metal Dichalcogenides by KEIT & Samsung Display

Here is a very interesting report from Korea Evaluation Institute of Industrial Technology (KEIT) supported by Samsung Display Co., Ltd. on self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides in the form of MoS2 for potential use in future display technology. The team was able to manufacture Large-area (~9 cm) mono-, bi-, and tri-layer MoS2 on a SiO2 substrate comparable in size to a cellular phone display screen.

Large-area (~9 cm) mono-, bi-, and tri-layer MoS2 on a SiO2 substrate comparable in size to a cellular phone display screen. (Picture From: Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides, licensed under a Creative Commons Attribution 4.0 International License.)

As you all know ALD is a self-limiting growth method,however in this case where growth occurs by the formation of multi-layer islands it is difficult to achieve the layer controllability needed when compared to CVD.  That is why the research team states three important findings for ALD growth of 2D layer structured materials:

  • Maximizing the self-limiting behavior of the ALD process to achieve layer controllability needed for a 2D structure by careful optimization of the process conditions (e.g., temperature, pressure, exposure of precursor/reactant)
  • Careful selection of the precursor and reactant - in this study MoCl5 and H2S are used as the precursors.
  • Understand the surface characteristics of the material being deposited.

Read all about it in the excellent open source report in published early this year in Scientific Reports:

Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides 

Youngjun Kim, Jeong-Gyu Song, Yong Ju Park, Gyeong Hee Ryu, Su Jeong Lee, Jin Sung Kim, Pyo Jin Jeon, Chang Wan Lee, Whang Je Woo, Taejin Choi, Hanearl Jung, Han-Bo-Ram Lee, Jae-Min Myoung, Seongil Im, Zonghoon Lee, Jong-Hyun Ahn, Jusang Park& Hyungjun Kim

Scientific Reports 6, Article number: 18754 (2016), doi:10.1038/srep18754

This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

Thursday, January 7, 2016

The Critical Materials Council to be managed by TECHCET in 2016

 The Critical Materials Council for Semiconductor Fabricators, originally established by ISMI/SEMATECH in the early 1990’s, will be managed by TECHCET CA LLC starting January 01, 2016. Under its new name CMC Fabs, the membership-based organization of semiconductor fab & fabless manufacturers will continue working to identify and remediate issues impacting the supply, availability, and accessibility of both current and emerging semiconductor process materials. In keeping with SEMATECH tradition, the work of the international council takes place in a non-competitive environment for the benefit of the semi device fabrication community. Topics addressed are identified and prioritized by the member companies.



The organization has a new website at cmcfabs.org, which includes an overview of the Council’s mission, news of upcoming events and a Members Only portal for access to minutes of monthly phone/WebEx meetings and workshop details. The site also features access for Members to the TECHCET Critical Materials Reports and the related quarterly updates.

The next face-to-face meeting of CMC Fabs will take place May 3-6, 2016 in Hillsboro, Oregon. The meeting will include the annual CMC Materials Seminar held on May 5-6 that is open to the public. Sessions include a market briefing, supply chain issues and methods, the evolution of emerging materials in ALD / ALE, and the materials revolution around carbon. Speakers will be drawn from fabs, suppliers and analysts to address topics of concern and interest to the Council, and the semiconductor materials supply chain.


CMC Fabs is a unit of TECHCET CA LLC, a firm focused on Process Materials Supply Chains, Electronic Materials Technology, Materials Market Research and Consulting for the Semiconductor, Display, Solar/PV, and LED Industries. The company has been responsible for producing the SEMATECH Critical Material Reports since 2000.

Thursday, December 17, 2015

Northwetsern and Argonne have synthesized Borophene

Borophene is a proposed crystalline allotrope of boron. Computational studies suggested that extended borophene sheets with partially filled hexagonal holes are stable. Borophene is predicted to be fully metallic and is analogous to graphene in that it is expected to form extended sheets. The latter is a semi-metal, implying that borophene may be a better conductor. The boron-boron bond is also nearly as strong as graphene’s carbon-carbon bond. [Wikipedia]
Borophene sheets. Image: Argonne National Laboratory
Now researchers at Northwestern University and Argonne National Laboratory have found an easy and inexpensive technique  to create borophene. Please check out the full details in the Science paper below.

Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs

Andrew J. Mannix et al
ScienceVol. 350 no. 6267 pp. 1513-1516, DOI: 10.1126/science.aad1080 

At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes. Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.