Continuous Tuning of Band Gap for π-Conjugated Ni Bis(dithiolene) Complex Bilayer

Shojaei, Fazel,Kang, Hong Seok American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.44

<P>Based on density functional calculations in combination with empirical corrections for the van der Waals interaction, we show that a recently synthesized two-dimensional sheet of π-conjugated Ni bis(dithiolene) complex (NiC<SUB>4</SUB>S<SUB>4</SUB>) exists in the form of bilayers with specific pairs of interlayer covalent bonds rather than in the form of isolated single layers. When one of the layers is slid relative to the other in the direction diagonal to two primitive vectors by applying a shear stress of ∼135 pN between the two layers brings about a gradual decrease in the band gap from 0.15 to 0 eV. Unlike in the case of an isolated single layer, the band gap of the bilayer is always found to be direct during the change. Therefore, the (NiC<SUB>4</SUB>S<SUB>4</SUB>)<SUB>2</SUB> bilayer will be quite useful in nanoelectromechanical devices as well as in optoelectronic devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-44/jp509462d/production/images/medium/jp-2014-09462d_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp509462d'>ACS Electronic Supporting Info</A></P>

Partially planar BP3with high electron mobility as a phosphorene analog

Shojaei, Fazel,Kang, Hong Seok Royal Society of Chemistry 2017 Journal of Materials Chemistry C Vol.5 No.43

<P>Based on first-principles calculations, we propose a two-dimensional α BP3crystal in which one quarter of the phosphorus atoms in phosphorene are substituted by boron atoms. Its geometrical structure resembles that of graphene more than that of phosphorene, although phosphorus is present as a major component. It is dynamically stable and energetically much more stable than another allotrope analogous to blue (β) phosphorene. The α BP3monolayer is a semiconductor with an indirect band gap (=0.77 eV) within the HSE06 calculation. For the BP3bilayer, three stacking patterns are equally stable within 3 meV per atom. The monolayer exhibits a high electron mobility (∼4.6 × 10<SUP>4</SUP>cm<SUP>2</SUP>V<SUP>−1</SUP>s<SUP>−1</SUP>) along the zigzag direction. In the bilayer, the mobility is ∼200 times higher than that of the α phosphorene bilayer. Specifically, it is even higher (∼3.7 × 10<SUP>5</SUP>cm<SUP>2</SUP>V<SUP>−1</SUP>s<SUP>−1</SUP>) and amounts to ∼2500 times in one of the patterns. Therefore, the BP3monolayer or bilayer will be useful as an n-type material in nanoelectronics through appropriate doping.</P>

Effect of Si–Si Bonds in Silicon-Doped α-Phosphorene Bilayers: Two-Dimensional Layers and One-Dimensional Nanoribbons

Shojaei, Fazel,Hahn, Jae Ryang,Kang, Hong Seok American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.30

<P>We investigate the geometrical and electronic structures of various configurations of 2Si-doped two-dimensional (2D) bilayers of black phosphorene (alpha P)(2), in which two P atoms are substituted by Si atoms. Our first-principles calculations suggest that doping is cooperative, which is clearly manifested in the formation of Si-Si bonds in the two most stable configurations. As a result, both configurations become indirect-gap semiconductors, which differ from that of the pristine 2D bilayer. On the one hand, 2Si-doped armchair phosphorene nanoribbon (APNR) bilayers possess pseudodirect band gaps in the most stable configuration, which are one-dimensional materials cut with armchair edges saturated with hydrogen atoms. Comparisons of the deformation energy and the activation barrier suggest that Stone-Wales (SW) deformation can occur substantially more easily in the doped APNR than in carbon nanotubes, and molecular dynamics simulations show that the SW defect will be kinetically stable. This is because the deformation brings about shortening and strengthening of weak Si-Si bonds. As a result, the APNRs turn into real direct-gap materials.</P>

Electronic structure and photocatalytic band offset of few-layer GeP2

Shojaei, Fazel,Hahn, Jae Ryang,Kang, Hong Seok Royal Society of Chemistry 2017 Journal of Materials Chemistry A Vol.5 No.42

<P>Based on a sophisticated crystal structure prediction method, we propose two-dimensional (2D) GeP2in the tetragonal (T) phase never observed for other group IV-V compounds. The bulk of 2D GeP2is more stable than both 2D orthogonal (O) and three-dimensional pyrite (P) phases that have been experimentally observed for group IV-V compounds. According to our calculations of phonon dispersion relations and elastic constants, as well as<I>ab initio</I>molecular dynamics simulation, monolayers of both the T and O phases (penta-GeP2and O-GeP2, respectively) are dynamically, mechanically, and thermally stable. In addition, our HSE06 calculation shows that these monolayers are semiconductors with band gaps in the visible region. Among the various stacking patterns of their bilayers, specific ones are identified to be most stable, which are still semiconductors with band gaps redshifted in the visible region. Different from the case of their bulk, few-layers of O-GeP2are more stable than those of penta-GeP2up to a pentalayer. Furthermore, band offset with respect to the Fermi levels of appropriate half-reactions shows that both n-type few-layer penta-GeP2and O-GeP2can be useful in photocatalyzed CO2splitting to CO as well as in photocatalyzed water splitting, specifically under acidic conditions.</P>

Mechanical and Electronic Properties of π-Conjugated Metal Bis(dithiolene) Complex Sheets

Shojaei, Fazel,Hahn, Jae Ryang,Kang, Hong Seok American Chemical Society 2014 Chemistry of materials Vol.26 No.9

<P>Using first-principles calculations, we have investigated the mechanical properties and electronic structures of π<SUP>–</SUP>conjugated metal bis(dithiolene) complex sheets (MC<SUB>4</SUB>S<SUB>4</SUB>), where M = Ni and Pd. First, the sheets are much softer than graphene due to their large porosity. At zero strain, NiC<SUB>4</SUB>S<SUB>4</SUB> is a semiconductor with an indirect gap, while PdC<SUB>4</SUB>S<SUB>4</SUB> is a metal. Under either biaxial or uniaxial strain, our band structure analysis demonstrates that the band gap of the NiC<SUB>4</SUB>S<SUB>4</SUB> slowly decreases to zero with increased strain, which can be attributed to the gradual weakening of π-bonds of the sheet. However, the PdC<SUB>4</SUB>S<SUB>4</SUB> becomes a magnetic system beyond the deformation threshold that causes a plastic deformation along the <I>X</I>-axis. In addition, we also observe that both two-dimensional sheets undergo different types of nonreversible plastic changes under the uniaxial strains along the <I>X</I>- and <I>Y</I>-axes.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2014/cmatex.2014.26.issue-9/cm500767u/production/images/medium/cm-2014-00767u_0011.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm500767u'>ACS Electronic Supporting Info</A></P>

Thickness-dependent bandgap and electrical properties of GeP nanosheets

Kim, Doyeon,Park, Kidong,Shojaei, Fazel,Debela, Tekalign Terfa,Kwon, Ik Seon,Kwak, In Hye,Seo, Jaemin,Ahn, Jae Pyoung,Park, Jeunghee,Kang, Hong Seok The Royal Society of Chemistry 2019 Journal of Materials Chemistry A Vol.7 No.27

<P>Recently there have been extensive efforts to develop novel two-dimensional (2D) layered structures, owing to their fascinating thickness-dependent optical/electrical properties. Herein, we synthesized thin GeP nanosheets that had a band gap (<I>E</I>g) of 2.3 eV, which is a dramatic increase from the value in the bulk (0.9 eV) upon exfoliation. This <I>E</I>g value is close to that of the GeP monolayer predicted by first-principles calculations (HSE06 functional). The calculations also indicate a strong dependence of <I>E</I>g on the number of layers (2.306, 1.660, 1.470, and 1.397 eV for mono-, bi-, tri-, and tetralayers, respectively), and that the band edge positions are suitable for water splitting reactions. Field-effect transistor devices were fabricated using the p-type GeP nanosheets of various thicknesses, and the devices demonstrated a significant decrease in the hole mobility but an increased on-off ratio as the layer number decreased. The larger on-off ratio (10<SUP>4</SUP>) for the thinner ones is promising for use in novel 2D (photo)electronic nanodevices. Further, liquid-exfoliated GeP nanosheets (thickness = 1-2 nm) deposited on Si nanowire arrays can function as a promising photoanode for solar-driven water-splitting photoelectrochemical (PEC) cells. Based on the calculated band offset with respect to the Fermi levels for the two half-reactions in the water splitting reaction, the performance of the PEC cell can be explained by the formation of an effective p-GeP/n-Si heterojunction.</P>

Arsenic for high-capacity lithium- and sodium-ion batteries

Lim, Young Rok,Shojaei, Fazel,Park, Kidong,Jung, Chan Su,Park, Jeunghee,Cho, Won Il,Kang, Hong Seok The Royal Society of Chemistry 2018 Nanoscale Vol.10 No.15

<P>We report arsenic (As) as a promising alternative to graphite anode materials in lithium- and sodium-ion batteries (LIBs and SIBs). The electrochemical properties of the As/carbon nanocomposite for both LIBs and SIBs were investigated using experimental and theoretical approaches. The LIBs showed excellent cycling performance, with a reversible capacity of 1306 mA h g<SUP>−1</SUP> (after 100 cycles), which is much higher than that of Li3As (1072 mA h g<SUP>−1</SUP>). In the corresponding SIBs, the measured reversible capacity was 750 mA h g<SUP>−1</SUP> (after 200 cycles), which is lower than that of Na3As. Extensive first-principles calculations were performed employing a structure prediction method for crystalline LixAs and NaxAs (<I>x</I> = 1-6) phases, as well as <I>ab initio</I> molecular dynamics simulations for their amorphous phases. In good agreement with the experimental LIB data, our calculations successfully predict the discharge capacity <I>versus</I> voltage curves, showing that the capacity of the amorphous phase reaches up to that of Li4As. In contrast, the SIB exhibited difficulty in reaching the predicted capacity (<I>x</I> = 3.5), probably due to significant volume expansion. Comparison of the theoretical discharge curves with the experimental data provides valuable information for the development of high-performance LIBs and SIBs.</P>

Red-to-Ultraviolet Emission Tuning of Two-Dimensional Gallium Sulfide/Selenide

Jung, Chan Su,Shojaei, Fazel,Park, Kidong,Oh, Jin Young,Im, Hyung Soon,Jang, Dong Myung,Park, Jeunghee,Kang, Hong Seok American Chemical Society 2015 ACS NANO Vol.9 No.10

<P>Graphene-like two-dimensional (2D) nanostructures have attracted significant attention because of their unique quantum confinement effect at the 2D limit. Multilayer nanosheets of GaS–GaSe alloy are found to have a band gap (<I>E</I><SUB>g</SUB>) of 2.0–2.5 eV that linearly tunes the emission in red-to-green. However, the epitaxial growth of monolayers produces a drastic increase in this <I>E</I><SUB>g</SUB> to 3.3–3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the <I>E</I><SUB>g</SUB> of these GaS and GaSe monolayers should be 3.325 and 3.001 eV, respectively. As the number of layers is increased to three, both the direct/indirect <I>E</I><SUB>g</SUB> decrease significantly; the indirect <I>E</I><SUB>g</SUB> approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect <I>E</I><SUB>g</SUB> of GaS to converge, resulting in monolayers with a strong emission. This wide <I>E</I><SUB>g</SUB> tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-10/acsnano.5b04876/production/images/medium/nn-2015-04876x_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b04876'>ACS Electronic Supporting Info</A></P>

Photoluminescence and Photocurrents of GaS_1–<i>x</i>Se_<i>x</i> Nanobelts

Jung, Chan Su,Park, Kidong,Shojaei, Fazel,Oh, Jin Young,Im, Hyung Soon,Lee, Jung Ah,Jang, Dong Myung,Park, Jeunghee,Myoung, NoSung,Lee, Chang-Lyoul,Lee, Jong Woon,Song, Jae Kyu,Kang, Hong Seok American Chemical Society 2016 Chemistry of materials Vol.28 No.16

<P>Two-dimensional layered structures have recently drawn worldwide attention because of their intriguing optical and electrical properties. GaS and GaSe are attractive layered materials owing to their wide band gap. Herein, we synthesized GaS1-xSex belt-type multilayers (nanobelts) with uniform morphology ([2110] hexagonal-phase long axis) by a chemical vapor transport method, and investigate their composition-dependent optical and optoelectronic properties. The GaS1-xSex exhibited strong visible-range photoluminescence at 490-620 nm (2.0-2.5 eV), with a unique composition dependence: longer decay time for the S-rich compositions (x <= 0.5). Photocurrent measurements were performed on individual nanobelts by fabricating photodetector devices; higher photocurrents were found for x <= 0.5. First-principles calculations predicted that oxygen chemisorption can cause the direct and indirect band gaps of GaS to converge, similar to the band structures of GaSe, and thus enhance the optical properties. On the basis of the band alignment (predicted by calculation) for the Schottky barriers in the metal-semiconductor metal photodetector, we proposed the origin of the higher photocurrent for GaS than for GaSe.</P>

Two-dimensional GeAs with a visible range band gap

Jung, Chan Su,Kim, Doyeon,Cha, Seunghwan,Myung, Yoon,Shojaei, Fazel,Abbas, Hafiz Ghulam,Lee, Jung Ah,Cha, Eun Hee,Park, Jeunghee,Kang, Hong Seok Royal Society of Chemistry 2018 Journal of Materials Chemistry A Vol.6 No.19

<P>Two-dimensional (2D) layered structures have recently drawn worldwide attention because of their intriguing optical and electrical properties. In this study, we prepared GeAs nanosheets as a new 2D material by a liquid-phase exfoliation method. The few-layered nanosheets had a band gap close to 2.1 eV, which is significantly higher (by about 1.5 eV) compared to the bulk. The value of 2.1 eV is in excellent agreement with that for the monolayer obtained from first-principles (HSE-06) calculations; mono-, bi-, tri-, and tetra-layers have remarkable direct or quasi-direct band gaps of 2.125, 1.339, 1.112, and 1.017 eV, respectively. The electrical properties of individual GeAs nanosheets were measured to reveal their 2D carrier transport behaviors. We also observed stable and large photocurrents, indicating potential application in high-performance optoelectronic nanodevices. The few-layered GeAs nanosheets deposited on n-type Si nanowire arrays showed promising photoelectrochemical water splitting under visible light irradiation. Band alignment based on the calculated band edge positions suggested a buildup of the space charge region in the p-GeAs/n-Si heterojunction, as well as the band bending of n-Si at the electrolyte interface.</P>