Publications by authors named "Zhizhan Qiu"

13 Publications

  • Page 1 of 1

Visualizing atomic structure and magnetism of 2D magnetic insulators via tunneling through graphene.

Nat Commun 2021 Jan 4;12(1):70. Epub 2021 Jan 4.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore.

The discovery of two-dimensional (2D) magnetism combined with van der Waals (vdW) heterostructure engineering offers unprecedented opportunities for creating artificial magnetic structures with non-trivial magnetic textures. Further progress hinges on deep understanding of electronic and magnetic properties of 2D magnets at the atomic scale. Although local electronic properties can be probed by scanning tunneling microscopy/spectroscopy (STM/STS), its application to investigate 2D magnetic insulators remains elusive due to absence of a conducting path and their extreme air sensitivity. Here we demonstrate that few-layer CrI (FL-CrI) covered by graphene can be characterized electronically and magnetically via STM by exploiting the transparency of graphene to tunneling electrons. STS reveals electronic structures of FL-CrI including flat bands responsible for its magnetic state. AFM-to-FM transition of FL-CrI can be visualized through the magnetic field dependent moiré contrast in the dI/dV maps due to a change of the electronic hybridization between graphene and spin-polarised CrI bands with different interlayer magnetic coupling. Our findings provide a general route to probe atomic-scale electronic and magnetic properties of 2D magnetic insulators for future spintronics and quantum technology applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-020-20376-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7782518PMC
January 2021

Printable two-dimensional superconducting monolayers.

Nat Mater 2021 Feb 26;20(2):181-187. Epub 2020 Oct 26.

Department of Chemistry, National University of Singapore, Singapore, Singapore.

Two-dimensional superconductor (2DSC) monolayers with non-centrosymmetry exhibit unconventional Ising pair superconductivity and an enhanced upper critical field beyond the Pauli paramagnetic limit, driving intense research interest. However, they are often susceptible to structural disorder and environmental oxidation, which destroy electronic coherence and provide technical challenges in the creation of artificial van der Waals heterostructures (vdWHs) for devices. Herein, we report a general and scalable synthesis of highly crystalline 2DSC monolayers via a mild electrochemical exfoliation method using flexible organic ammonium cations solvated with neutral solvent molecules as co-intercalants. Using NbSe as a model system, we achieved a high yield (>75%) of large-sized single-crystal monolayers up to 300 µm. The as-fabricated, twisted NbSe vdWHs demonstrate high stability, good interfacial properties and a critical current that is modulated by magnetic field when one flux quantum fits to an integer number of moiré cells. Additionally, formulated 2DSC inks can be exploited to fabricate wafer-scale 2D superconducting wire arrays and three-dimensional superconducting composites with desirable morphologies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41563-020-00831-1DOI Listing
February 2021

Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction.

Nat Commun 2020 Sep 1;11(1):4389. Epub 2020 Sep 1.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore.

The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped AuPt clusters supported on defective graphene. This creates a bimetal single cluster catalyst (AuPt/G) with exceptional activity for electrochemical nitrogen (N) reduction. Our mechanistic study reveals that each N molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N via an enhanced back donation of electrons to the N LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-020-18080-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7463028PMC
September 2020

Real-Space Imaging of a Single-Molecule Monoradical Reaction.

J Am Chem Soc 2020 Aug 17;142(31):13550-13557. Epub 2020 Jul 17.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.

Organic radicals consisting of light elements exhibit a low spin-orbit coupling and weak hyperfine interactions with a long spin coherence length, which are crucial for future applications in molecular spintronics. However, the synthesis and characterization of these organic radicals have been a formidable challenge due to their chemical instability arising from unpaired electrons. Here, we report a direct imaging of the surface chemical transformation of an organic monoradical synthesized via the monodehydrogenation of a chemically designed precursor. Bond-resolved scanning tunneling microscopy unambiguously resolves various products formed through a complex structural dissociation and rearrangement of organic monoradicals. Density functional theory calculations reveal detailed reaction pathways from the monoradical to different cyclized products. Our study provides unprecedented insights into complex surface reaction mechanisms of organic radical reactions at the single molecule level, which may guide the design of stable organic radicals for future quantum technology applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c05337DOI Listing
August 2020

Semimetal or Semiconductor: The Nature of High Intrinsic Electrical Conductivity in TiS.

J Phys Chem Lett 2019 Nov 31;10(22):6996-7001. Epub 2019 Oct 31.

Department of Physics, Applied Physics, and Astronomy , Rensselaer Polytechnic Institute , Troy , New York 12180 , United States.

As an intensively studied electrode material for secondary batteries, TiS is known to exhibit high electrical conductivity without extrinsic doping. However, the origin of this high conductivity, either being a semimetal or a heavily self-doped semiconductor, has been debated for several decades. Here, combining quasi-particle GW calculations, density functional theory (DFT) study on intrinsic defects, and scanning tunneling microscopy/spectroscopy (STM/STS) measurements, we conclude that stoichiometric TiS is a semiconductor with an indirect band gap of about 0.5 eV. The high conductivity of TiS is therefore caused by heavy self-doping. Our DFT results suggest that the dominant donor defect that is responsible for the self-doping under thermal equilibrium is Ti interstitial, which is corroborated by our STM/STS measurements.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.9b02710DOI Listing
November 2019

Strain-Induced Isomerization in One-Dimensional Metal-Organic Chains.

Angew Chem Int Ed Engl 2019 Dec 4;58(51):18591-18597. Epub 2019 Nov 4.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore.

The ability to use mechanical strain to steer chemical reactions creates completely new opportunities for solution- and solid-phase synthesis of functional molecules and materials. However, this strategy is not readily applied in the bottom-up on-surface synthesis of well-defined nanostructures. We report an internal strain-induced skeletal rearrangement of one-dimensional (1D) metal-organic chains (MOCs) via a concurrent atom shift and bond cleavage on Cu(111) at room temperature. The process involves Cu-catalyzed debromination of organic monomers to generate 1,5-dimethylnaphthalene diradicals that coordinate to Cu adatoms, forming MOCs with both homochiral and heterochiral naphthalene backbone arrangements. Bond-resolved non-contact atomic force microscopy imaging combined with density functional theory calculations showed that the relief of substrate-induced internal strain drives the skeletal rearrangement of MOCs via 1,3-H shifts and shift of Cu adatoms that enable migration of the monomer backbone toward an energetically favorable registry with the Cu(111) substrate. Our findings on this strain-induced structural rearrangement in 1D systems will enrich the toolbox for on-surface synthesis of novel functional materials and quantum nanostructures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.201909074DOI Listing
December 2019

Atomically precise bottom-up synthesis of π-extended [5]triangulene.

Sci Adv 2019 Jul 26;5(7):eaav7717. Epub 2019 Jul 26.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.

The zigzag-edged triangular graphene molecules (ZTGMs) have been predicted to host ferromagnetically coupled edge states with the net spin scaling with the molecular size, which affords large spin tunability crucial for next-generation molecular spintronics. However, the scalable synthesis of large ZTGMs and the direct observation of their edge states have been long-standing challenges because of the molecules' high chemical instability. Here, we report the bottom-up synthesis of π-extended [5]triangulene with atomic precision via surface-assisted cyclodehydrogenation of a rationally designed molecular precursor on metallic surfaces. Atomic force microscopy measurements unambiguously resolve its ZTGM-like skeleton consisting of 15 fused benzene rings, while scanning tunneling spectroscopy measurements reveal edge-localized electronic states. Bolstered by density functional theory calculations, our results show that [5]triangulenes synthesized on Au(111) retain the open-shell π-conjugated character with magnetic ground states.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1126/sciadv.aav7717DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6660211PMC
July 2019

Giant gate-tunable bandgap renormalization and excitonic effects in a 2D semiconductor.

Sci Adv 2019 Jul 19;5(7):eaaw2347. Epub 2019 Jul 19.

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore.

Understanding the remarkable excitonic effects and controlling the exciton binding energies in two-dimensional (2D) semiconductors are crucial in unlocking their full potential for use in future photonic and optoelectronic devices. Here, we demonstrate large excitonic effects and gate-tunable exciton binding energies in single-layer rhenium diselenide (ReSe) on a back-gated graphene device. We used scanning tunneling spectroscopy and differential reflectance spectroscopy to measure the quasiparticle electronic and optical bandgap of single-layer ReSe, respectively, yielding a large exciton binding energy of 520 meV. Further, we achieved continuous tuning of the electronic bandgap and exciton binding energy of monolayer ReSe by hundreds of milli-electron volts through electrostatic gating, attributed to tunable Coulomb interactions arising from the gate-controlled free carriers in graphene. Our findings open a new avenue for controlling the bandgap renormalization and exciton binding energies in 2D semiconductors for a wide range of technological applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1126/sciadv.aaw2347DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6641939PMC
July 2019

Tailoring sample-wide pseudo-magnetic fields on a graphene-black phosphorus heterostructure.

Nat Nanotechnol 2018 09 25;13(9):828-834. Epub 2018 Jun 25.

Department of Chemistry, National University of Singapore, Singapore, Singapore.

Spatially tailored pseudo-magnetic fields (PMFs) can give rise to pseudo-Landau levels and the valley Hall effect in graphene. At an experimental level, it is highly challenging to create the specific strain texture that can generate PMFs over large areas. Here, we report that superposing graphene on multilayer black phosphorus creates shear-strained superlattices that generate a PMF over an entire graphene-black phosphorus heterostructure with edge size of tens of micrometres. The PMF is intertwined with the spatial period of the moiré pattern, and its spatial distribution and intensity can be modified by changing the relative orientation of the two materials. We show that the emerging pseudo-Landau levels influence the transport properties of graphene-black phosphorus field-effect transistor devices with Hall bar geometry. The application of an external magnetic field allows us to enhance or reduce the effective field depending on the valley polarization with the prospect of developing a valley filter.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41565-018-0178-zDOI Listing
September 2018

Defects controlled hole doping and multivalley transport in SnSe single crystals.

Nat Commun 2018 01 3;9(1):47. Epub 2018 Jan 3.

Department of Physics, Zhejiang University, Hangzhou, 310027, China.

SnSe is a promising thermoelectric material with record-breaking figure of merit. However, to date a comprehensive understanding of the electronic structure and most critically, the self-hole-doping mechanism in SnSe is still absent. Here we report the highly anisotropic electronic structure of SnSe investigated by angle-resolved photoemission spectroscopy, in which a unique pudding-mould-shaped valence band with quasi-linear energy dispersion is revealed. We prove that p-type doping in SnSe is extrinsically controlled by local phase segregation of SnSe microdomains via interfacial charge transferring. The multivalley nature of the pudding-mould band is manifested in quantum transport by crystallographic axis-dependent weak localisation and exotic non-saturating negative magnetoresistance. Strikingly, quantum oscillations also reveal 3D Fermi surface with unusual interlayer coupling strength in p-SnSe, in which individual monolayers are interwoven by peculiar point dislocation defects. Our results suggest that defect engineering may provide versatile routes in improving the thermoelectric performance of the SnSe family.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-017-02566-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5752673PMC
January 2018

Resolving the Spatial Structures of Bound Hole States in Black Phosphorus.

Nano Lett 2017 11 20;17(11):6935-6940. Epub 2017 Oct 20.

Department of Chemistry, National University of Singapore , 3 Science Drive 3, Singapore 117543.

Understanding the local electronic properties of individual defects and dopants in black phosphorus (BP) is of great importance for both fundamental research and technological applications. Here, we employ low-temperature scanning tunnelling microscope (LT-STM) to probe the local electronic structures of single acceptors in BP. We demonstrate that the charge state of individual acceptors can be reversibly switched by controlling the tip-induced band bending. In addition, acceptor-related resonance features in the tunnelling spectra can be attributed to the formation of Rydberg-like bound hole states. The spatial mapping of the quantum bound states shows two distinct shapes evolving from an extended ellipse shape for the 1s ground state to a dumbbell shape for the 2p excited state. The wave functions of bound hole states can be well-described using the hydrogen-like model with anisotropic effective mass, corroborated by our theoretical calculations. Our findings not only provide new insight into the many-body interactions around single dopants in this anisotropic two-dimensional material but also pave the way to the design of novel quantum devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.7b03356DOI Listing
November 2017

Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus.

Nano Lett 2017 03 16;17(3):1970-1977. Epub 2017 Feb 16.

Department of Chemistry, National University of Singapore , 3 Science Drive 3, Singapore 117543.

Two-dimensional black phosphorus (BP) has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic properties. BP is one of the few two-dimensional materials where it is possible to tune the bandgap over a wide energy range from the visible up to the infrared. In this article, we report the observation of a giant Stark effect in electrostatically gated few-layer BP. Using low-temperature scanning tunnelling microscopy, we observed that in few-layer BP, when electrons are injected, a monotonic reduction of the bandgap occurs. The injected electrons compensate the existing defect-induced holes and achieve up to 35.5% bandgap modulation in the light-doping regime. When probed by tunnelling spectroscopy, the local density of states in few-layer BP shows characteristic resonance features arising from layer-dependent sub-band structures due to quantum confinement effects. The demonstration of an electrical gate-controlled giant Stark effect in BP paves the way to designing electro-optic modulators and photodetector devices that can be operated in a wide electromagnetic spectral range.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.6b05381DOI Listing
March 2017
-->