From DOE’s Brookhaven National Laboratory (US) : “Zhongwei Dai- Exploring the Strange Quantum World of 2D Materials” 

From DOE’s Brookhaven National Laboratory (US)

October 21, 2021
Ariana Manglaviti
amanglaviti@bnl.gov

The Center for Functional Nanomaterials [below] researcher uses electrons and light to probe the properties of atomically thin materials to identify promising candidates for quantum information science applications.

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Zhongwei Dai performs low-energy electron microscopy and diffraction at a Center for Functional Nanomaterials (CFN)-operated experimental endstation of the Electron Spectro-Microscopy beamline at the National Synchrotron Light Source II [below]. Using these electron-based techniques, Dai probes the properties of atomically thin quantum materials such as graphene.

In 2010, The University of Manchester (UK) researchers won the Nobel Prize in Physics for their 2004 discovery of graphene—a form of carbon only one atom thick, about one-millionth the thickness of a strand of hair. The accidental discovery of the world’s first two-dimensional (2D) material was based on a surprisingly simple method involving a common office supply: Scotch tape. With the sticky tape, the research group had been peeling off the topmost layer of graphite—the main component of pencils—to expose a clean surface for calibrating a microscope used to image surfaces at the atomic scale. One day, they noticed black residue on used tape in a trash bin and decided to investigate further. After repeated peeling, they eventually isolated a single layer of carbon atoms in crystalline form. Graphene proved to have remarkable electrical, mechanical, and chemical properties, including high electrical conductivity, durability, and transparency.

­At the time, Zhongwei Dai had just earned his bachelor’s in physics and was about to start his PhD program in condensed matter physics and materials science at The University of New Hampshire (US). As someone who grew up observing the world around him and trying to understand how things work—such as how a flame in a fire gets its shape—Dai knew he wanted to study the physics of materials. When he heard the graphene discovery story, he decided to become a 2D quantum materials scientist.

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A single layer of graphene (left). Stacked layers of graphene (right) form bulk graphite (top).

“I was amazed by how they found a new material with the most basic tools instead of the expensive, fancy equipment scientific research typically requires,” said Dai, who, for the past three years, has been a postdoctoral researcher in the Interface Science and Catalysis Group of the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory. “I became excited about the other 2D materials out there waiting to be discovered and explored. These materials have the potential to revolutionize our world.”

From user to collaborator to postdoc

Dai’s first interaction with the CFN was as a user while he was completing his PhD work. For his research, he used the CFN x-ray photoemission electron microscopy/low-energy electron microscopy (XPEEM/LEEM) endstation at the Electron Spectro-Microscopy beamline at the National Synchrotron Light Source II (NSLS-II) [below]. Here, he performed dynamical low-energy electron diffraction (LEED) to study the different surface atomic structures of various 2D crystals, including graphene; transition metal dichalcogenides (TMDs) such as molybdenum diselenide and tungsten ditelluride; and transition metal tri-halides like ruhenium trichloride.

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A top view of the atomic structure of an individual chlorine (red)-ruthenium (gray), chlorine (green) sandwich layer. Research published in 2D Materials 7, no. 3 (2020).

In dynamical LEED, the energy of the electrons bombarding the sample is tuned and the intensity of the electrons scattered back is recorded, producing an intensity vs voltage (IV) spectrum. Dai co-wrote software to analyze the data produced by the XPEEM/LEEM endstation. He compared the experimental spectrum with a calculated spectrum based on an atomic model of how atoms may be arranged on the surface. Through an iterative process, he refined the atomic model to see which atomic arrangement produced a simulated spectrum most closely matching the experimental one. Dai discovered each of the 2D crystals had a slightly different surface structure compared to their “parent” 3D crystals. Dai also modernized a LEED-IV calculation package to more quickly provide crystal information; this program was installed on the CFN computing cluster.

Brookhaven Lab CFN Computing Cluster

After getting to know some of the CFN staff—including materials scientist Jerzy Sadowksi, who oversees the XPEEM/LEEM endstation—Dai began collaborating with them on 2D materials research. For example, they determined the surface structure of ruthenium trichloride, a magnetic material of interest for quantum applications.

As Dai prepared to graduate from UNH, he applied to an open CFN postdoc position, which would be supervised by Sadowski. He was selected, joining the CFN in October 2018.

Tools of the trade

As a CFN postdoc, Dai has been studying the fundamental behavior of electrons and light in different 2D quantum materials—including graphene, TMDs, and hexagonal boron nitride (an insulator)—in hopes of identifying promising candidates for quantum information science (QIS) applications.

“Because of their reduced dimensionality, 2D materials have different and peculiar properties that may be useful for making next-generation electronic or optoelectronic devices,” said Dai.

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LEEM (a and b) was used to locate the sample area of interest, followed by dynamical LEED acquired at different electron energies (c, d, and e), to investigate the crystalline structure of molybdenum disulfide on a silicon substrate. The first LEEM image (a) shows bulk (more than 100 layers thick), monolayer (1 ML), bilayer (2 ML), and trilayer (3 ML) molybdenum disulfide. The second LEEM image (b) shows areas where circular holes were drilled and covered with 1 ML of molybdenum sulfide; without a substrate underneath, the 1 ML is “suspended.” The dynamical LEED patterns are of bulk molybdenum disulfide, with c and e clearly displaying three-fold symmetry. Research published in Surface Science, 660 (2017).

To study these materials, Dai primarily employs three complementary surface-sensitive techniques: Raman spectroscopy, LEEM, and LEED. While LEEM and LEED shoot electrons at a sample, Raman spectroscopy shines light onto a sample. Based on how the light gets scatterd back, Raman spectra provide distinct chemical fingerprints of materials and their crystal phases and defects. The Raman spectrometer Dai uses is attached to a laser-based optical microscope. Part of the robotic Quantum Material Press (QPress) under development at the CFN, this confocal Raman microscope can probe light-matter interactions at targeted sample areas.

“By combining imaging and spectroscopy, the microscope gives us detailed information about how chemical components are distributed and how crystallinity varies in the material,” said Dai. “I initially check the crystallinity of 2D crystals through confocal Raman spectroscopy because it’s fast—it operates outside of a vacuum chamber, so I can easily go to different spots on the sample and collect a spectrum to see whether those areas are good crystals or not.”

Next, Dai takes photos of the crystal surface with LEEM to determine its cleanliness. He ultimately checks the crystallinity with LEED, which is sensitive to a material’s periodic structure, or how the atoms are regularly arranged.

“These are complementary techniques relying on two fundamentally different elemental particles as probes: light and electrons,” said Dai.

Another tool Dai uses is a scattering-type scanning near-field optical microscope. This microscope measures the near-field optical signals between a scanning probe tip with a laser beam shone onto it and a material surface. A 2D image of the surface is generated based on these measurements, with the image contrast capturing local interactions between light and atoms on the surface. In this way, Dai can study surface quantum phenomena.

A quest for useful 2D materials

Recently, Dai, Sadowski, and collaborators from the CFN and The University of Pennsylvania (US) studied bilayer graphene in which one of the two layers was rotated by 30 degrees relative to the other. As they reported in Physical Review Letters, they found the distance between the two layers increased in this twisted configuration relative to a stacked one, where the layers are stacked directly on top of each other. This increased interlayer spacing for electrons to move could lead to devices with interesting and unexpected properties.

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Dai looks through one of the windows into the central QPress chamber, which contains the robot for routing samples. The smaller white dots appearing on each of these windows are part of a self-check alignment system to determine whether the robot arm is at that location.

Dai has also been building and studying quantum heterostructures, refering to individual 2D materials stacked together to form entirey new materials with never-before-seen properties. The CFN QPress is designed to synthesize and characterize these composite materials. On the synthesis side, two challenges are determining which adhesive tape is the best to peel atomic layers from parent crystals and how to make the 2D heterostructure interfaces interactive.

“The QPress uses sticky polymers for exfoliation, and residue from the polymers may stay on the 2D crystals and influence the interface interactions,” explained Dai. “Also, if contaminants like water and other small molecules get sandwiched in between the materials, the interfacial interactions will be impaired or disappear. LEEM can detect these contaminants.”

For his bilayer graphene research, Dai created intrinsically clean 2D surfaces by growing the materials using chemical vapors and heating the materials in ultrahigh vacuum. However, this chemical growth method can only make specific configurations (in that case, 30-degrees-rotated bilayer graphene). Heterostructures with other relative rotation angles—such as the “magic” angle of 1.1 degrees in bilayer graphene reported to induce superconductivity, or the frictionless flow of electrons—can only be made by artifically stacking the materials, as the QPress does.

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The QPress exfoliator.

“When you put different 2D materials together, the possibilities are endless,” said Dai.

Of all possible heterostructures, graphene on top of ruthenium trichloride is one Dai is particularly interested in. From an electrical conductivity standpoint, these two materials are opposites; graphene is a semi-metal, whereas ruthenium trichloride is an insulator.

“At this interface, the electrons are under the influence of both a conducting and nonconducting material,” said Dai. “It’s a peculiar spot where I suspect we can gain meaningful insights about how electrons move, not only in this system but also in others.”

Technology revolution

With Dai’s postdoc at the CFN coming to an end, he is appreciative of his experience.

“The world-class faciliites and expert staff scientists at the CFN gave me an opportunity to advance my research that nowhere else in the world can provide,” said Dai. “I especially benefitted from working with not only my research group but also the other groups across the CFN—Soft and Bio Nanomaterials for photoluminesence spectroscopy to study the electronic structure of my materials, Electronic Nanomaterials for making insulating QPress substrates conductive through thin films of titanium oxide for electron-based techniques, Electron Microscopy for imaging these films, and Theory and Computation for providing theoretical insights to help me understand my experimental results. Without CFN Director Chuck Black encouraging this synergy, I don’t think my research would have been possible. I’m also grateful for all the help I received from support staff.”

Next, Dai will seek a faculty position in which he can continue his research on 2D quantum materials, with an eye toward both fundamental processes and real-world applications.

“I want to collaborate with the device community to make prototype devices with the 2D heterostructure materials I study, completing the feedback loop in the search for new quantum technologies.”

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One of ten national laboratories overseen and primarily funded by the DOE(US) Office of Science, DOE’s Brookhaven National Laboratory (US) conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University(US), the largest academic user of Laboratory facilities, and Battelle(US), a nonprofit, applied science and technology organization.

Research at BNL specializes in nuclear and high energy physics, energy science and technology, environmental and bioscience, nanoscience and national security. The 5,300 acre campus contains several large research facilities, including the Relativistic Heavy Ion Collider [below] and National Synchrotron Light Source II [below]. Seven Nobel prizes have been awarded for work conducted at Brookhaven lab.

BNL is staffed by approximately 2,750 scientists, engineers, technicians, and support personnel, and hosts 4,000 guest investigators every year. The laboratory has its own police station, fire department, and ZIP code (11973). In total, the lab spans a 5,265-acre (21 km^2) area that is mostly coterminous with the hamlet of Upton, New York. BNL is served by a rail spur operated as-needed by the New York and Atlantic Railway. Co-located with the laboratory is the Upton, New York, forecast office of the National Weather Service.

Major programs

Although originally conceived as a nuclear research facility, Brookhaven Lab’s mission has greatly expanded. Its foci are now:

Nuclear and high-energy physics
Physics and chemistry of materials
Environmental and climate research
Nanomaterials
Energy research
Nonproliferation
Structural biology
Accelerator physics

Operation

Brookhaven National Lab was originally owned by the Atomic Energy Commission(US) and is now owned by that agency’s successor, the United States Department of Energy (DOE). DOE subcontracts the research and operation to universities and research organizations. It is currently operated by Brookhaven Science Associates LLC, which is an equal partnership of Stony Brook University(US) and Battelle Memorial Institute(US). From 1947 to 1998, it was operated by Associated Universities, Inc. (AUI) (US), but AUI lost its contract in the wake of two incidents: a 1994 fire at the facility’s high-beam flux reactor that exposed several workers to radiation and reports in 1997 of a tritium leak into the groundwater of the Long Island Central Pine Barrens on which the facility sits.

Foundations

Following World War II, the US Atomic Energy Commission was created to support government-sponsored peacetime research on atomic energy. The effort to build a nuclear reactor in the American northeast was fostered largely by physicists Isidor Isaac Rabi and Norman Foster Ramsey Jr., who during the war witnessed many of their colleagues at Columbia University leave for new remote research sites following the departure of the Manhattan Project from its campus. Their effort to house this reactor near New York City was rivalled by a similar effort at the Massachusetts Institute of Technology (US) to have a facility near Boston, Massachusettes(US). Involvement was quickly solicited from representatives of northeastern universities to the south and west of New York City such that this city would be at their geographic center. In March 1946 a nonprofit corporation was established that consisted of representatives from nine major research universities — Columbia University(US), Cornell University(US), Harvard University(US), Johns Hopkins University(US), Massachusetts Institute of Technology(US), Princeton University(US), University of Pennsylvania(US), University of Rochester(US), and Yale University(US).

Out of 17 considered sites in the Boston-Washington corridor, Camp Upton on Long Island was eventually chosen as the most suitable in consideration of space, transportation, and availability. The camp had been a training center from the US Army during both World War I and World War II. After the latter war, Camp Upton was deemed no longer necessary and became available for reuse. A plan was conceived to convert the military camp into a research facility.

On March 21, 1947, the Camp Upton site was officially transferred from the U.S. War Department to the new U.S. Atomic Energy Commission (AEC), predecessor to the U.S. Department of Energy (DOE).

Research and facilities

Reactor history

In 1947 construction began on the first nuclear reactor at Brookhaven, the Brookhaven Graphite Research Reactor. This reactor, which opened in 1950, was the first reactor to be constructed in the United States after World War II. The High Flux Beam Reactor operated from 1965 to 1999. In 1959 Brookhaven built the first US reactor specifically tailored to medical research, the Brookhaven Medical Research Reactor, which operated until 2000.

Accelerator history

In 1952 Brookhaven began using its first particle accelerator, the Cosmotron. At the time the Cosmotron was the world’s highest energy accelerator, being the first to impart more than 1 GeV of energy to a particle.

BNL Cosmotron 1952-1966

The Cosmotron was retired in 1966, after it was superseded in 1960 by the new Alternating Gradient Synchrotron (AGS).

BNL Alternating Gradient Synchrotron (AGS)

The AGS was used in research that resulted in 3 Nobel prizes, including the discovery of the muon neutrino, the charm quark, and CP violation.

In 1970 in BNL started the ISABELLE project to develop and build two proton intersecting storage rings.

The groundbreaking for the project was in October 1978. In 1981, with the tunnel for the accelerator already excavated, problems with the superconducting magnets needed for the ISABELLE accelerator brought the project to a halt, and the project was eventually cancelled in 1983.

The National Synchrotron Light Source (US) operated from 1982 to 2014 and was involved with two Nobel Prize-winning discoveries. It has since been replaced by the National Synchrotron Light Source II (US) [below].

BNL National Synchrotron Light Source (US).

After ISABELLE’S cancellation, physicist at BNL proposed that the excavated tunnel and parts of the magnet assembly be used in another accelerator. In 1984 the first proposal for the accelerator now known as the Relativistic Heavy Ion Collider (RHIC)[below] was put forward. The construction got funded in 1991 and RHIC has been operational since 2000. One of the world’s only two operating heavy-ion colliders, RHIC is as of 2010 the second-highest-energy collider after the Large Hadron Collider(CH). RHIC is housed in a tunnel 2.4 miles (3.9 km) long and is visible from space.

On January 9, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of Energy Office of Science, that the BNL eRHIC design has been selected over the conceptual design put forward by DOE’s Thomas Jefferson National Accelerator Facility [Jlab] (US) as the future Electron–ion collider (EIC) in the United States.

Brookhaven Lab Electron-Ion Collider (EIC) (US) to be built inside the tunnel that currently houses the RHIC.

In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 from the Department of Energy. BNL’s eRHIC design proposes upgrading the existing Relativistic Heavy Ion Collider, which collides beams light to heavy ions including polarized protons, with a polarized electron facility, to be housed in the same tunnel.

Other discoveries

In 1958, Brookhaven scientists created one of the world’s first video games, Tennis for Two. In 1968 Brookhaven scientists patented Maglev, a transportation technology that utilizes magnetic levitation.

Major facilities

Relativistic Heavy Ion Collider (RHIC), which was designed to research quark–gluon plasma and the sources of proton spin. Until 2009 it was the world’s most powerful heavy ion collider. It is the only collider of spin-polarized protons.

Center for Functional Nanomaterials (CFN), used for the study of nanoscale materials.

BNL National Synchrotron Light Source II(US), Brookhaven’s newest user facility, opened in 2015 to replace the National Synchrotron Light Source (NSLS), which had operated for 30 years. NSLS was involved in the work that won the 2003 and 2009 Nobel Prize in Chemistry.

Alternating Gradient Synchrotron, a particle accelerator that was used in three of the lab’s Nobel prizes.
Accelerator Test Facility, generates, accelerates and monitors particle beams.
Tandem Van de Graaff, once the world’s largest electrostatic accelerator.

Computational Science resources, including access to a massively parallel Blue Gene series supercomputer that is among the fastest in the world for scientific research, run jointly by Brookhaven National Laboratory and Stony Brook University-SUNY (US).

Interdisciplinary Science Building, with unique laboratories for studying high-temperature superconductors and other materials important for addressing energy challenges.
NASA Space Radiation Laboratory, where scientists use beams of ions to simulate cosmic rays and assess the risks of space radiation to human space travelers and equipment.

Off-site contributions

It is a contributing partner to the ATLAS experiment, one of the four detectors located at the Large Hadron Collider (LHC).

European Organization for Nuclear Research (Organisation européenne pour la recherche nucléaire)(EU) [CERN] map

Iconic view of the European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire](CH)CERN ATLAS detector.

It is currently operating at The European Organization for Nuclear Research [Organisation européenne pour la recherche nucléaire] [Europäische Organisation für Kernforschung](CH) [CERN] near Geneva, Switzerland.

Brookhaven was also responsible for the design of the SNS accumulator ring in partnership with Spallation Neutron Source at DOE’s Oak Ridge National Laboratory (US), Tennessee.

DOE’s Oak Ridge National Laboratory(US) Spallation Neutron Source annotated.

Brookhaven plays a role in a range of neutrino research projects around the world, including the Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China.

Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China.

FNAL DUNE LBNF (US) from FNAL to Sanford Underground Research Facility, Lead, South Dakota, USA.

Brookhaven Campus.

BNL Center for Functional Nanomaterials.

BNL National Synchrotron Light Source II(US).

BNL NSLS II (US).

BNL Relative Heavy Ion Collider (US) Campus.

BNL/RHIC Star Detector.

BNL/RHIC Phenix detector.