Modifying the Band Gap of Semiconducting Two-Dimensional Materials by Polymer Assembly into Different Structures

Abstract

Polyethylene glycol (PEG) assembled on the surface of two-dimensional tungsten disulfide (WS 2 ) into a limited number of nanoislands (NIs), nanoshells (NSs), and granular nanoparticulates (GNPs) depending on its chain length. NI assemblies showed a nonmeasurable shift of photoluminescence (PL) and the A and B absorption peaks of WS 2 . This confirmed that the electronic doping by thiol is not effective. The PEG NS assembly displayed a smaller red shift of the PL and a slight decrease of the energy difference between the A and B absorption peaks of WS 2 . However, increasing the dielectric function on the surface of WS 2 has a small influence on their optical properties. The PEG NP assembly on WS 2 exhibited a significant red shift of the PL spectrum and a large decrease of the energy difference between A and B absorption peaks. Deforming the WS 2 sheet by the PEG NP assembly decreased the orbital coupling and lowered the electronic direct band gap significantly. Raman bands of WS 2 are shifted to a higher frequency on improving its mechanical strength after the PEG assembly.

Tuning the Optical Band Gap of Two-Dimensional WS2 Integrated with Gold Nanocubes by Introducing Palladium Nanostructures

“…The energy of optical A and B peaks of WS 2 is modulated by changing the SOC, which can be tuned by (1) applying an external magnetic field that alters the SOC, thus changing the energy of the valence and conduction bands; (2) changing the crystal structure from 2H to 1T phase, which alters the positions of the S and W atoms inside the sheet and consequentially the SOC; (3) applying strain that disturbs the bond lengths and the SOC; and (4) increasing the charge density on the surface of WS 2 sheets by electron injection from touching electronic systems, thus disturbing the SOC. − …”

“…WS 2 nanosheets are characterized by two Raman bands, one corresponding to out-of-plane vibration appears at high frequency, A 1 ′ band, and a low-frequency E ′ band arises from the in-plane vibration band. , The E ′ band is overlapped with the second-order longitudinal acoustic phonons band (2LA) at the edge of the Brillouin zone (M point) of WS 2 . The point group is changed from D 6 h symmetry for bulk into D 3 h and D 3 d for 2-D WS 2 when the number of layers is odd or even, respectively.…”

Metal Nanoclusters Modify the Band Gap and Maintain the Ultrathin Nature of Semiconducting Two-Dimensional Materials

“…The energy difference between the A and B absorption peaks is 0.176 eV. The A and B absorption peak positions of MoS 2 sheets and the energy difference between them depend on the SOC of the 4d electrons of Mo and 3p electrons of S. The SOC can be changed by electron injection from rich electronic systems, changing the crystal structures from the thermodynamically favorable 2H phase to 1T phase by temperature, applying a magnetic field, and mechanical deformation of the sheet. ,,,,, …”

“…Semiconducting two-dimensional materials (S2DM) exhibit band gap values and band edges, which are well suited for many optoelectronic applications such as photodiodes and phototransistors, solar cells, photocatalytic hydrogen generation, and photoelectrochemical sensors with ultrathin conformation. − Monolayered molybdenum disulfide (MoS 2 ) is constructed by two hexagonal planes of sulfur atoms separated by a hexagonal plane of molybdenum atoms in a trigonal prismatic structure . Unlike the bulk crystal, which is characterized by its indirect band gap due to Γ to K transition, single- and few-layer MoS 2 demonstrate a direct optical band gap of ∼1.99 eV at the K point of the Brillouin zone. , The electronic transition in single-layer MoS 2 can be transformed from direct to indirect band gap upon applying strain . The energy of the valence band maximum (VBM) at the Γ point increases by increasing the strain, thus overtaking the VBM at the K point. − This tunable band gap of MoS 2 is because of the changes of the spin-orbital coupling (SOC) and orbital interactions between the Mo and S atoms induced by strain. − The optical band gap of S2DM can be tuned by changing the dielectric constant of the surrounding, combining with another S2DM, changing temperature, electron or charge transfer doping, and applying a magnetic field. − Engineering the band gap of S2DM is essential when tailoring the design of ultrathin devices based on them.…”

“…12,13 The electronic transition in single-layer MoS 2 can be transformed from direct to indirect band gap upon applying strain. 14 The energy of the valence band maximum (VBM) at the Γ point increases by increasing the strain, thus overtaking the VBM at the K point. 15−17 This tunable band gap of MoS 2 is because of the changes of the spin-orbital coupling (SOC) and orbital interactions between the Mo and S atoms induced by strain.…”