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The linear positive magnetoresistance (MR) and negative MR are emerging as intriguing phenomena in topological materials. Here, we report the magnetotransport properties of topological insulator Bi2Se3 flakes with different thicknesses grown by chemical vapor deposition. A non-saturating positive MR is observed in the perpendicular fields (B⊥I), while an evident negative MR is observed in the parallel fields (B//I). Both the positive MR and negative MR show a linear magnetic field dependence at high magnetic fields (B > 8 T). In addition, two types of carriers are extracted from the nonlinear Hall resistance measurements, which are attributed to the Bi2Se3 bulk electrons and the electron accumulation layer at the surface or Bi2Se3/SiO2 interface, respectively. The conductivity fluctuations in the accumulation layer are believed to be the origin for the observation of positive MR in the perpendicular fields and negative MR in the parallel fields with linear and non-saturating characteristics.

The linear positive magnetoresistance (MR) and negative MR are emerging as intriguing phenomena in topological materials. Here, we report the magnetotransport properties of topological insulator Bi2Se3 flakes with different thicknesses grown by chemical vapor deposition. A non-saturating positive MR is observed in the perpendicular fields (B⊥I), while an evident negative MR is observed in the parallel fields (B//I). Both the positive MR and negative MR show a linear magnetic field dependence at high magnetic fields (B > 8 T). In addition, two types of carriers are extracted from the nonlinear Hall resistance measurements, which are attributed to the Bi2Se3 bulk electrons and the electron accumulation layer at the surface or Bi2Se3/SiO2 interface, respectively. The conductivity fluctuations in the accumulation layer are believed to be the origin for the observation of positive MR in the perpendicular fields and negative MR in the parallel fields with linear and non-saturating characteristics.

The linear positive magnetoresistance (MR) and negative MR are emerging as intriguing phenomena in topological materials. Here, we report the magnetotransport properties of topological insulator Bi2Se3 flakes with different thicknesses grown by chemical vapor deposition. A non-saturating positive MR is observed in the perpendicular fields (B⊥I), while an evident negative MR is observed in the parallel fields (B//I). Both the positive MR and negative MR show a linear magnetic field dependence at high magnetic fields (B > 8 T). In addition, two types of carriers are extracted from the nonlinear Hall resistance measurements, which are attributed to the Bi2Se3 bulk electrons and the electron accumulation layer at the surface or Bi2Se3/SiO2 interface, respectively. The conductivity fluctuations in the accumulation layer are believed to be the origin for the observation of positive MR in the perpendicular fields and negative MR in the parallel fields with linear and non-saturating characteristics.

Binary topological insulator Bi2X3 (X=Te, Se) nanoplates have been synthesised via catalyst free chemical vapor deposition method. A silicon nitride protecting layer was deposited after the growth to minimize the damage in Raman spectra measurements of these nanoplates. Temperature dependent non-resonant Raman spectra with the excitation of 514.5 nm show rather small first-order temperature coefficients for all Raman modes in Bi2Te3 and Bi2Se3 nanoplates compared to the reported value in bulk single crystal. Besides, the A1g2 mode shows an electronic resonant enhancement in Raman spectra with the excitation of 633 nm, which also has a small first-order temperature coefficient as the non-resonant ones. Magnetotransport measurements for grown Bi2Se3 nanoplates show pronounced weak anti-localization effect in the low B-field region, linear magnetoresistance in the perpendicular high B-field and a weak negative magnetoresistance in the in-plane direction. These magnetotransport features are considered to originate from the topological surface states. Ternary compound Bi2Te1.7Se1.3, Bi2TeSe2 and Bi2.1Te1.4Se1.5 nanoplates have been deposited on SiO2/Si substrates via catalyst free chemical vapor deposition method. The evolution of Raman spectra for Bi2(Te1-xSex)3 has been studied, which shows blue shift in A1g1, Eg2 and A1g2 mode when increase the atomic ratio of Se. A new Raman mode (V) appears in the ternary compound compared to the binary compound, we attempt to attribute this new mode to the Te1-Se2 anti-sites in Bi2(Te1-xSex)3.

Superconducting proximity effect in topological insulator and superconductor hybrid structure has attracted intense attention in recent years in an effort to search for Majorana fermions in condensed matter systems. Here we report on the superconducting proximity effect in a Bi2Se3/NbSe2 junction fabricated with an all-dry transfer method. Experimentally measured differential conductance spectra exhibit a bias-independent conductance plateau (BICP) in the vicinity of zero bias below 7 K and a zero-bias conductance peak (ZBCP) appears below 2 K. We show that the BICP is due to the strong superconducting proximity effect between the superconductor and the topological surface states while the ZBCP is due to the bulk states of Bi2Se3. Our study gives direct evidence that the topological surface states can be strongly coupled to a superconductor and clarifies the different roles of surface states and bulk states in superconducting proximity effect.

We report the systemic magneto transport studies on a-few-layers exfoliated WTe2 thin film devices. The non-saturating and large magnetoresistance (MR), proportional to B2, are observed in all devices. Hall effect modeling using the classical two-band model indicate the nearly identical electron and hole densities in thicker devices while unequal densities of electrons and holes in thinner devices at low temperatures. As a result, we attribute the large non-saturating MR at low temperatures in thicker devices to balanced electron and hole density while in thinner devices to low mobility caused by surface contamination and degradation. Our results suggest both carrier densities and mobility play significant roles in determining the magnitude and the non-saturating behavior of MR.

Superconducting proximity effect in topological insulator and superconductor hybrid structure has attracted intense attention in recent years in an effort to search for Majorana fermions in condensed matter systems. Here we report on the superconducting proximity effect in a Bi2Se3/NbSe2 junction fabricated with an all-dry transfer method. Experimentally measured differential conductance spectra exhibit a bias-independent conductance plateau (BICP) in the vicinity of zero bias below 7 K and a zero-bias conductance peak (ZBCP) appears below 2 K. We show that the BICP is due to the strong superconducting proximity effect between the superconductor and the topological surface states while the ZBCP is due to the bulk states of Bi2Se3. Our study gives direct evidence that the topological surface states can be strongly coupled to a superconductor and clarifies the different roles of surface states and bulk states in superconducting proximity effect.

We report the systemic magneto transport studies on a-few-layers exfoliated WTe2 thin film devices. The non-saturating and large magnetoresistance (MR), proportional to B2, are observed in all devices. Hall effect modeling using the classical two-band model indicate the nearly identical electron and hole densities in thicker devices while unequal densities of electrons and holes in thinner devices at low temperatures. As a result, we attribute the large non-saturating MR at low temperatures in thicker devices to balanced electron and hole density while in thinner devices to low mobility caused by surface contamination and degradation. Our results suggest both carrier densities and mobility play significant roles in determining the magnitude and the non-saturating behavior of MR.

Superconducting proximity effect in topological insulator and superconductor hybrid structure has attracted intense attention in recent years in an effort to search for Majorana fermions in condensed matter systems. Here we report on the superconducting proximity effect in a Bi2Se3/NbSe2 junction fabricated with an all-dry transfer method. Experimentally measured differential conductance spectra exhibit a bias-independent conductance plateau (BICP) in the vicinity of zero bias below 7 K and a zero-bias conductance peak (ZBCP) appears below 2 K. We show that the BICP is due to the strong superconducting proximity effect between the superconductor and the topological surface states while the ZBCP is due to the bulk states of Bi2Se3. Our study gives direct evidence that the topological surface states can be strongly coupled to a superconductor and clarifies the different roles of surface states and bulk states in superconducting proximity effect.

We report the systemic magneto transport studies on a-few-layers exfoliated WTe2 thin film devices. The non-saturating and large magnetoresistance (MR), proportional to B2, are observed in all devices. Hall effect modeling using the classical two-band model indicate the nearly identical electron and hole densities in thicker devices while unequal densities of electrons and holes in thinner devices at low temperatures. As a result, we attribute the large non-saturating MR at low temperatures in thicker devices to balanced electron and hole density while in thinner devices to low mobility caused by surface contamination and degradation. Our results suggest both carrier densities and mobility play significant roles in determining the magnitude and the non-saturating behavior of MR.

Binary topological insulator Bi2X3 (X=Te, Se) nanoplates have been synthesised via catalyst free chemical vapor deposition method. A silicon nitride protecting layer was deposited after the growth to minimize the damage in Raman spectra measurements of these nanoplates. Temperature dependent non-resonant Raman spectra with the excitation of 514.5 nm show rather small first-order temperature coefficients for all Raman modes in Bi2Te3 and Bi2Se3 nanoplates compared to the reported value in bulk single crystal. Besides, the A1g2 mode shows an electronic resonant enhancement in Raman spectra with the excitation of 633 nm, which also has a small first-order temperature coefficient as the non-resonant ones. Magnetotransport measurements for grown Bi2Se3 nanoplates show pronounced weak anti-localization effect in the low B-field region, linear magnetoresistance in the perpendicular high B-field and a weak negative magnetoresistance in the in-plane direction. These magnetotransport features are considered to originate from the topological surface states. Ternary compound Bi2Te1.7Se1.3, Bi2TeSe2 and Bi2.1Te1.4Se1.5 nanoplates have been deposited on SiO2/Si substrates via catalyst free chemical vapor deposition method. The evolution of Raman spectra for Bi2(Te1-xSex)3 has been studied, which shows blue shift in A1g1, Eg2 and A1g2 mode when increase the atomic ratio of Se. A new Raman mode (V) appears in the ternary compound compared to the binary compound, we attempt to attribute this new mode to the Te1-Se2 anti-sites in Bi2(Te1-xSex)3.

Two-dimensional (2D) graphene oxide/polypyrrole (GO/PPy) hybrid materials derived from in-situ polymerization are used as precursors for constructing functionalized three-dimensional (3D) porous nitrogen-doped carbon nanosheet frameworks (FT-PNCNFs) through a one-step activation strategy. In the formation process of FT-PNCNFs, PPY is directly converted into hierarchical porous nitrogen-doped carbon layers, while GO is simultaneously reduced to become electrically conductive. The complementary functions of individual components endow the FT-PNCNFs with excellent properties for both supercapacitors (SCs) and sodium ion batteries (SIBs) applications. When tested in symmetrical SC, the FT-PNCNFs demonstrate superior energy storage behaviour. At an extremely high scan rate of 3000 mV s-1, the cyclic voltammetry (CV) curve retains an inspiring quasi-rectangle shape in KOH solution. Meanwhile, high capacitances (∼247 F g-1 at 10 mV s-1; ∼146 F g-1 at 3000 mV s-1) and good cycling stability (∼95% retention after 8000 cycles) are achieved. In addition, an attractive SIB anode performance could be achieved. The FT-PNCNFs electrode delivers a reversible capacity of 187 mAh g-1 during 160th cycle at 100 mA g-1. Its reversible capacity retains 144 mAh g-1 after extending the number of cycles to 500 at 500 mA g-1.

Two-dimensional (2D) graphene oxide/polypyrrole (GO/PPy) hybrid materials derived from in-situ polymerization are used as precursors for constructing functionalized three-dimensional (3D) porous nitrogen-doped carbon nanosheet frameworks (FT-PNCNFs) through a one-step activation strategy. In the formation process of FT-PNCNFs, PPY is directly converted into hierarchical porous nitrogen-doped carbon layers, while GO is simultaneously reduced to become electrically conductive. The complementary functions of individual components endow the FT-PNCNFs with excellent properties for both supercapacitors (SCs) and sodium ion batteries (SIBs) applications. When tested in symmetrical SC, the FT-PNCNFs demonstrate superior energy storage behaviour. At an extremely high scan rate of 3000 mV s-1, the cyclic voltammetry (CV) curve retains an inspiring quasi-rectangle shape in KOH solution. Meanwhile, high capacitances (∼247 F g-1 at 10 mV s-1; ∼146 F g-1 at 3000 mV s-1) and good cycling stability (∼95% retention after 8000 cycles) are achieved. In addition, an attractive SIB anode performance could be achieved. The FT-PNCNFs electrode delivers a reversible capacity of 187 mAh g-1 during 160th cycle at 100 mA g-1. Its reversible capacity retains 144 mAh g-1 after extending the number of cycles to 500 at 500 mA g-1.

Two-dimensional (2D) graphene oxide/polypyrrole (GO/PPy) hybrid materials derived from in-situ polymerization are used as precursors for constructing functionalized three-dimensional (3D) porous nitrogen-doped carbon nanosheet frameworks (FT-PNCNFs) through a one-step activation strategy. In the formation process of FT-PNCNFs, PPY is directly converted into hierarchical porous nitrogen-doped carbon layers, while GO is simultaneously reduced to become electrically conductive. The complementary functions of individual components endow the FT-PNCNFs with excellent properties for both supercapacitors (SCs) and sodium ion batteries (SIBs) applications. When tested in symmetrical SC, the FT-PNCNFs demonstrate superior energy storage behaviour. At an extremely high scan rate of 3000 mV s-1, the cyclic voltammetry (CV) curve retains an inspiring quasi-rectangle shape in KOH solution. Meanwhile, high capacitances (∼247 F g-1 at 10 mV s-1; ∼146 F g-1 at 3000 mV s-1) and good cycling stability (∼95% retention after 8000 cycles) are achieved. In addition, an attractive SIB anode performance could be achieved. The FT-PNCNFs electrode delivers a reversible capacity of 187 mAh g-1 during 160th cycle at 100 mA g-1. Its reversible capacity retains 144 mAh g-1 after extending the number of cycles to 500 at 500 mA g-1.

Two-dimensional (2D) graphene oxide/polypyrrole (GO/PPy) hybrid materials derived from in-situ polymerization are used as precursors for constructing functionalized three-dimensional (3D) porous nitrogen-doped carbon nanosheet frameworks (FT-PNCNFs) through a one-step activation strategy. In the formation process of FT-PNCNFs, PPY is directly converted into hierarchical porous nitrogen-doped carbon layers, while GO is simultaneously reduced to become electrically conductive. The complementary functions of individual components endow the FT-PNCNFs with excellent properties for both supercapacitors (SCs) and sodium ion batteries (SIBs) applications. When tested in symmetrical SC, the FT-PNCNFs demonstrate superior energy storage behaviour. At an extremely high scan rate of 3000 mV s-1, the cyclic voltammetry (CV) curve retains an inspiring quasi-rectangle shape in KOH solution. Meanwhile, high capacitances (∼247 F g-1 at 10 mV s-1; ∼146 F g-1 at 3000 mV s-1) and good cycling stability (∼95% retention after 8000 cycles) are achieved. In addition, an attractive SIB anode performance could be achieved. The FT-PNCNFs electrode delivers a reversible capacity of 187 mAh g-1 during 160th cycle at 100 mA g-1. Its reversible capacity retains 144 mAh g-1 after extending the number of cycles to 500 at 500 mA g-1.