Nanoribbon Heterojunctions Research Articles

Novel Two-Dimensional Mechano-Electric Generators and Sensors Based on Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, provide two-dimensional atomic crystals with semiconductor band gap. In this work, we present a design of new mechano-electric generators and sensors based on transition metal dichalcogenide nanoribbon PN junctions and heterojunctions. The mechano-electric conversion was simulated by using a first-principle calculation. The output voltage of MoS2 nanoribbon PN junction increases with strain, reaching 0.036 V at 1% strain and 0.31 V at 8% strain, much larger than the reported results. Our study indicates that the length, width and layer number of TMDC nanoribbon PN junctions have an interesting but different impact on the voltage output. Also, the results indicate that doping position and concentration only cause a small fluctuation in the output voltage. These results have been compared with the mechano-electric conversion of TMDC heterojunctions. Such novel mechano-electric generators and sensors are very attractive for applications in future self-powered, wearable electronics and systems.

Band gap engineering in finite elongated graphene nanoribbon heterojunctions: Tight-binding model

A simple model based on the divide and conquer rule and tight-binding (TB) approximation is employed for studying the role of finite size effect on the electronic properties of elongated graphene nanoribbon (GNR) heterojunctions. In our model, the GNR heterojunction is divided into three parts: a left (L) part, middle (M) part, and right (R) part. The left part is a GNR of width WL, the middle part is a GNR of width WM, and the right part is a GNR of width WR. We assume that the left and right parts of the GNR heterojunction interact with the middle part only. Under this approximation, the Hamiltonian of the system can be expressed as a block tridiagonal matrix. The matrix elements of the tridiagonal matrix are computed using real space nearest neighbor orthogonal TB approximation. The electronic structure of the GNR heterojunction is analyzed by computing the density of states. We demonstrate that for heterojunctions for which WL = WR, the band gap of the system can be tuned continuously by varying the length of the middle part, thus providing a new approach to band gap engineering in GNRs. Our TB results were compared with calculations employing divide and conquer rule in combination with density functional theory (DFT) and were found to agree nicely.

Spin-dependent Seebeck Effect, Thermal Colossal Magnetoresistance and Negative Differential Thermoelectric Resistance in Zigzag Silicene Nanoribbon Heterojunciton.

Spin-dependent Seebeck effect (SDSE) is one of hot topics in spin caloritronics, which examine the relationships between spin and heat transport in materials. Meanwhile, it is still a huge challenge to obtain thermally induced spin current nearly without thermal electron current. Here, we construct a hydrogen-terminated zigzag silicene nanoribbon heterojunction, and find that by applying a temperature difference between the source and the drain, spin-up and spin-down currents are generated and flow in opposite directions with nearly equal magnitudes, indicating that the thermal spin current dominates the carrier transport while the thermal electron current is much suppressed. By modulating the temperature, a pure thermal spin current can be achieved. Moreover, a thermoelectric rectifier and a negative differential thermoelectric resistance can be obtained in the thermal electron current. Through the analysis of the spin-dependent transport characteristics, a phase diagram containing various spin caloritronic phenomena is provided. In addition, a thermal magnetoresistance, which can reach infinity, is also obtained. Our results put forward an effective route to obtain a spin caloritronic material which can be applied in future low-power-consumption technology.

Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions.

Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling and solar energy conversion. However, the performance of such devices degrades as their size is reduced. Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale. Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry. It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon. Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.

Atomically Precise Graphene Nanoribbon Heterojunctions for Excitonic Solar Cells

By mixing pure precursor monomers and nitrogen-doped equivalents, atomically sharp wiggle-edged heterojunctions can be obtained via the combined action of Ullmann coupling followed by cyclodehydrogenation [Cai et al., Nat. Nanotechnol. 2014, 9, 896]. We used first-principles density functional theory and the many-body GW approach to establish the role of doping (boron and nitrogen) in a variety of graphene nanowiggles displaying a range of band gaps. The substitution of C atoms located at the edges of the structures does not significantly affect the magnitude of the band gaps, but leads to their relative upshift or downshift depending on the dopant. This shift is found to vary quasi-linearly as the concentration of dopant increases. Consequently, tunable type-II staggered band alignments are formed in graphene nanowiggle heterojunctions. We predict that these type-II heterojunctions can provide ultrathin solar cells with power conversion efficiencies up to 22.0%.

Graphene nanoribbon heterojunctions.

Despite graphene's remarkable electronic properties, the lack of an electronic bandgap severely limits its potential for applications in digital electronics. In contrast to extended films, narrow strips of graphene (called graphene nanoribbons) are semiconductors through quantum confinement, with a bandgap that can be tuned as a function of the nanoribbon width and edge structure. Atomically precise graphene nanoribbons can be obtained via a bottom-up approach based on the surface-assisted assembly of molecular precursors. Here we report the fabrication of graphene nanoribbon heterojunctions and heterostructures by combining pristine hydrocarbon precursors with their nitrogen-substituted equivalents. Using scanning probe methods, we show that the resulting heterostructures consist of seamlessly assembled segments of pristine (undoped) graphene nanoribbons (p-GNRs) and deterministically nitrogen-doped graphene nanoribbons (N-GNRs), and behave similarly to traditional p-n junctions. With a band shift of 0.5 eV and an electric field of 2 × 10(8) V m(-1) at the heterojunction, these materials bear a high potential for applications in photovoltaics and electronics.

Electronic transport properties of graphene nanoribbon heterojunctions with 5–7–5 ring defect

Using the non-equilibrium Green’s function method together with the density-functional theory, the electronic transport properties of heterojunction models constructed with armchair and zigzag graphene nanoribbon (GNR) units via topological defects have been investigated. It is found that the diode-like behavior and conductivity of the armchair9–zigzag4 (A9–Z4) junctions are associated with the length of the nanoribbon sub-units closely. The structure similar to the p–n junction is formed around the defect region, inducing the rectifying property of the model. Moreover, the conductivity of symmetrical heterojunction structures can be optimized by controlling the length of GNR units.

Tunable band gap of AlN, GaN nanoribbons and AlN/GaN nanoribbon heterojunctions: A first-principle study

We systematically investigate the electronic structure of AlN, GaN and (AlN)x/(GaN)1−x nanoribbon heterojunctions by employing first-principle calculations. The band gaps of both AlN and GaN ribbons decrease monotonically with the increase of the widths, and the ribbons with armchair edges are direct band gap semiconductors while the zigzag ones are indirect. Interestingly, the band gaps of (AlN)x/(GaN)1−x nanoribbon heterojunctions are closely associated with the AlN/GaN ratios and increase monotonically with the increase of the AlN concentration. The flexible tunability of the band gap in these nanostructures may be widely applied to future optoelectronic devices.

Electronic transport properties of graphyne and its family

Using the non-equilibrium Green’s function method together with the density-functional theory, the electronic transport properties of graphyne and its family have been studied. Unlike graphene, the graphyne and its family display semi-conductive characteristic along zigzag direction and metallic characteristic along armchair direction. The transport properties of graphyne and its family are associated with the length of C link in their structures. With the length of C link increasing, the electrical conductivity decreases. In addition, both the zigzag and armchair graphdiyne nanoribbons display semi-conductive characteristic. The armchair graphene–graphdiyne nanoribbon heterojunction displays symmetrical semi-conductive characteristic. However, the zigzag graphene–graphdiyne nanoribbon heterojunction shows asymmetrical metallic characteristic and displays the superior rectification behavior. Comparison with the previous studies, it can be found that the heterojunction constructed with metallic and semi-conductive nanoelements which behave distinct electronic structures can display the rectification behavior, and it can open up opportunities for design of nanodevices.

Rectifying performance in zigzag graphene nanoribbon heterojunctions with different edge hydrogenations

Abstract Using nonequilibrium Greenʼs functions in combination with the density functional theory, we investigated the electronic transport behaviors of zigzag graphene nanoribbon (ZGNR) heterojunctions with different edge hydrogenations. The results show that electronic transport properties of ZGNR heterojunctions can be modulated by hydrogenations, and prominent rectification effects can be observed. We propose that the edge dihydrogenation leads to a blocking of electronic transfer, as well as the changes of the distribution of the frontier orbital at negative/positive bias might be responsible for the rectification effects. These results may be helpful for designing practical devices based on graphene nanoribbons.

Spin Seebeck Effect and Thermal Colossal Magnetoresistance in Graphene Nanoribbon Heterojunction

Spin caloritronics devices are very important for future development of low-power-consumption technology. We propose a new spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of single-hydrogen-terminated ZGNR (ZGNR-H) and double-hydrogen-terminated ZGNR (ZGNR-H2). We predict that spin-up and spin-down currents flowing in opposite directions can be induced by temperature difference instead of external electrical bias. The thermal spin-up current is considerably large and greatly improved compared with previous work in graphene. Moreover, the thermal colossal magnetoresistance is obtained in our research, which could be used to fabricate highly-efficient spin caloritronics MR devices.

High magnetoresistance in graphene nanoribbon heterojunction

We show a large magnetoresistance (MR) effect in a graphene heterostructure consisting of a metallic and semiconductor-type armchair-graphene-nanoribbon. In the heterostructure, the transmission across the first subband of the semiconducting armchair-graphene-nanoribbon and metallic armchair-graphene-nanoribbon is forbidden under zero magnetic-field, due to the orthogonality of the wavefunctions. A finite magnetic-field introduces the quantum hall-like effect, which distorts the wavefunctions. Thus, a finite transmission occurs across the heterojunction, giving rise to a large MR effect. We study the dependence of this MR on temperature and electron energy. Finally, we design a magnetic-field-effect-transistor which yields a MR of close to 100% (85%) at low (room) temperature.

Orbital symmetry induced conductance switching in a graphene nanoribbon heterojunction with different edge hydrogenations

First principles calculations are performed to investigate the electron transport through a zigzag-edged graphene nanoribbon (ZGNR) heterojunction constructed by connecting a monohydrogenated ZGNR and a dihydrogenated ZGNR and its response to external magnetic fields. It is found that the heterojunction can be switched between a conducting state and an insulating state by tuning the magnetic fields. It arises from the matching or mismatching between the π or π* states of the two ribbons under different magnetic fields. This mechanism of conductance switching by tuning the orbital symmetry can be considered in the future design of graphene based electronic devices.

Influence of contact doping on graphene nanoribbon heterojunction tunneling field effect transistors

We investigated the device performance of graphene nanoribbon tunneling field-effect transistors with heterogeneous channel as a function of the contact doping concentrations. The simulations were carried out based on the non-equilibrium Green’s function, coupled with a Dirac Hamiltonian model, and the roles of symmetric and asymmetric contact doping concentrations on the device performance were identified. It was observed that the device performances such as OFF-state currents (IOFF), ON-state currents (ION) and subthreshold slopes (SSs) were greatly influenced by the source doping concentrations, while variations in drain doping concentrations changed mainly the IOFF. By applying proper asymmetric source and drain doping concentrations, low SS and large ION/IOFF ratio can be achieved, indicating that it is an alternative route to effectively enhance the device performance.

Intraribbon Heterojunction Formation in Ultranarrow Graphene Nanoribbons

Graphene nanoribbons-semiconducting quasi-one-dimensional graphene structures-have great potential for the realization of novel electronic devices. Recently, graphene nanoribbon heterojunctions-interfaces between nanoribbons with unequal band gaps-have been realized with lithographic etching techniques and via chemical routes to exploit quantum transport phenomena. However, standard fabrication techniques are not suitable for ribbons narrower than ~5 nm and do not allow to control the width and edge structure of a specific device with atomic precision. Here, we report the realization of graphene nanoribbon heterojunctions with lateral dimensions below 2 nm via controllable dehydrogenation of polyanthrylene oligomers self-assembled on a Au(111) surface from molecular precursors. Atomistic simulations reveal the microscopic mechanisms responsible for intraribbon heterojunction formation. We demonstrate the capability to selectively modify the heterojunctions by activating the dehydrogenation reaction on single units of the nanoribbons by electron injection from the tip of a scanning tunneling microscope.

Electronic transport through zigzag/armchair graphene nanoribbon heterojunctions

The electronic transport properties of a graphene nanoribbon (GNR) are known to be sensitive to its width, edges and defects. We investigate the electronic transport properties of a graphene nanoribbon heterojunction constructed by fusing a zigzag and an armchair graphene nanoribbon (zGNR/aGNR) side by side. First principles results reveal that the heterojunction can be either metallic or semiconducting, depending on the width of the nanoribbons. Intrinsic rectification behaviors have been observed, which are largely sensitive to the connection length between the zGNR and aGNR. The microscopic origins of the rectification behavior have been revealed. We find that the carrier type can alter from electrons to holes with the bias voltage changing from negative to positive; the asymmetrical transmission spectra of electrons and holes induced by the interface defects directly results in the rectification behavior. The results suggest that any methods which can enhance the asymmetry of the transmission spectra between holes and electrons could be used to improve the rectification behavior in the zGNR/aGNR heterojunction. Our findings could be useful for designing graphene based electronic devices.

Electronic and Magnetic Properties of Hybrid Graphene Nanoribbons with Zigzag-Armchair Heterojunctions

Inspired by the experimentally observed zigzag-armchair graphene nanoribbon heterojunctions, we constructed a new class of infinitely long hybrid graphene nanoribbons (HGNRs), and systematically investigated their electronic and magnetic properties by means of spin-polarized first-principles computations. HGNRs are converted from nonmagnetic semiconductors to magnetic semiconductors by increasing the length of zigzag segments. In particular, half metallicity can be achieved in HGNRs under external transverse electric fields. These results suggest that the introduction of armchair “impurity” will not affect the desired electronic and magnetic properties of zigzag graphene nanoribbons.

Edge Hydrogenation-Induced Spin-Filtering and Rectifying Behaviors in the Graphene Nanoribbon Heterojunctions

By using nonequilibrium Green’s functions in combination with the density functional theory, we investigate the electronic transport properties of edge hydrogenated zigzag-edged graphene nanoribbon heterojunctions. Results show that a perfect spin-filtering effect with 100% spin polarization and a rectifying behavior with a ratio larger than 105 can be realized by dihydrogenation, which can also be modulated by changing the widths of the two component ribbons.

Electronic structures of zigzag AlN, GaN nanoribbons and Al xGa 1− xN nanoribbon heterojunctions: First-principles study

The electronic and structural properties of zigzag aluminum nitride (AlN), gallium nitride (GaN) nanoribbons and Al x Ga 1− x N nanoribbon heterojunctions are investigated using the first-principles calculations. Both AlN and GaN ribbons are found to be semiconductor with an indirect band gap, which decreases monotonically with the increased ribbon width, and approaching to the gaps of their infinite two dimensional graphitic-like monolayer structures, respectively. Furthermore, the band gap of Al x Ga 1− x N nanoribbon heterojunctions is closely related to Al (and/or Ga) concentrations. The Al x Ga 1− x N nanoribbon of width n=8 shows a continuously band gap varying from about 2.2 eV–3.1 eV as x increases from 0 to 1. The large ranged tunable band gaps in such a quasi one dimension structure may open up new opportunities for these AlN/GaN based materials in future optoelectronic devices.

Atomistic quantum transport modeling of metal-graphene nanoribbon heterojunctions

We calculate quantum transport for metal-graphene nanoribbon heterojunctions within the atomistic self-consistent Schr\"odinger/Poisson scheme. Attention is paid on both the chemical aspects of the interface bonding as well the one-dimensional electrostatics along the ribbon length. Band-bending and doping effects strongly influence the transport properties, giving rise to conductance asymmetries and a selective suppression of the subband formation. Junction electrostatics and $p$-type characteristics drive the conduction mechanism in the case of high-work-function Au, Pd, and Pt electrodes while contact resistance becomes dominant in the case of Al.