1 Introduction
Graphene exfoliated from bulk graphite is the first two-dimensional material experimentally studied by scientists. It has more excellent properties than bulk graphite, so its discoverers Andre Geim and Konstantin Sergeevich Novoselov were awarded the 2010 Nobel Prize in Physics. After graphene, scientists have discovered thousands of two-dimensional materials, creating a new research field and research hotspot, attracting researchers in physics, chemistry, materials and other directions to conduct in-depth basic and applied research
[1].
In recent years, theoretical and experimental studies have shown that the electrical, optical, magnetic and other properties of two-dimensional materials will be significantly different from those of two-dimensional materials after they are made into one-dimensional nanoribbons.And these properties can be effectively controlled by the width and edge structure of the nanoribbons, so one-dimensional nanoribbons prepared from two-dimensional materials have become the research focus in the field of two-dimensional materials in recent years.
Taking graphene nanoribbons as an example, as shown in Figure 1, they can be classified into armchair, zigzag and other types according to the edge structure
[2]. Similarly, nanoribbons prepared from other two-dimensional materials can also be classified into armchair, zigzag, and other types. The confinement effect caused by the width direction and the defect-like effect caused by the edge structure make one-dimensional nanoribbons different from two-dimensional materials in properties and applications. At present, there is no article systematically introducing one-dimensional nanoribbons of two-dimensional materials. In this paper, the structure, properties and preparation methods of one-dimensional nanobelts of various two-dimensional materials are systematically introduced, and the advantages and disadvantages of different preparation methods are summarized, and several important research directions and application prospects that may make breakthroughs in the future are prospected.
2 Graphene nanoribbon
2.1 Structure and properties of graphene nanoribbons.
Graphene is a carbon nanomaterial with a two-dimensional honeycomb lattice structure
[3]. If the size of graphene is limited in a certain direction, a ribbon structure is formed, called graphene nanoribbon, which is another one-dimensional carbon nanorod structure besides carbon nanotubes and carbon chains
[4,5][5,6]. The minimum width of the graphene nanoribbon can be less than 1 nm; When the width is less than 10 nm, it has obvious size effect, which brings unique performance. For example, unlike zero-gap, non-magnetic graphene, both theory and experiment show that armchair graphene nanoribbons (AGNRs) have appreciable band gaps, up to more than 2.3 eV, and the size and width are inversely proportional
[7]. Theoretical calculations show that the band gap of N
a-AGNRs depends on the width (N
a is the number of double bonds in the width direction of AGNRs, which is a positive integer).
According to the size of band gap and its variation, it can be divided into three types: :N
a=3p, 3p + 1, 3p + 2 (p is a positive integer). As shown in Fig. 2: according to the tight-binding calculation, when N
a=3p+2, the AGNRs are metallic, and the other two are semiconducting; However, AGNRs with N
a=3p+2 are semiconductors with small band gap calculated by local density approximation, and have large band gap when N
a=3p or N
a=3p+1. The band gaps of the three types of AGNRs all decrease with increasing width, and the band gap values are Δ
3p+1>Δ
3p>
. The 5-armchair type also exhibits magnetic properties
[10].
For zigzag graphene nanoribbons (ZGNRs), the band gap is also width-dependent: it is zero when it is smaller than the critical width, and it is of the order of a hundred meV when it is larger than the critical width, and the critical width is generally a few nanometers (Figure 3A)
[11]. The electron spins at the left and right edges of ZGNRs are in opposite directions (Fig. 3 B). ZGNRs exhibit half-metallicity when an in-plane electric field is applied in the width direction. As shown in Figure 3C, one of the spins gradually opens its direct energy gap with increasing electric field, which is called α-spin (red line); The direct gap of the other spin decreases until it transforms into an indirect gap of magnitude 0, referred to as the β-spin (blue line). ZGNRs can be considered as a semimetal because the electrons with different spin directions are metallic or insulating under the electric field. In addition, the electron spin polarization of ZGNRs leads to the appearance of ferromagnetic coupling and antiferromagnetic coupling, showing a large magnetoresistance
[2].
2.2 Preparation of graphene nanoribbons.
The preparation methods of graphene nanoribbons are generally divided into two categories. One is the "top-down" method, which processes graphene or carbon nanotubes to obtain graphene nanoribbons. The other is the "bottom-up" method, which synthesizes graphene nanoribbons from precursor molecules. Several main preparation methods will be briefly introduced below. The method for preparing nanoribbons from other two-dimensional materials is similar.
2.2.1 Two-dimensional sheet etching
Traditional etching methods, such as electron beam/ion beam etching and dip-pen nano-etching, can obtain wider graphene nanoribbons, generally with a width of 10 ~ 100 nm. Although they can show a certain confinement effect, their properties are still close to those of graphene.
Han et al. Reported the preparation of graphene nanoribbons with various widths (width ranging from 10 to 100 nm, length ranging from 1 to 2 μm) and different crystal orientations by plasma etching
[12]. As shown in fig. 4A, graphene with a lateral size of about 20 μm was contacted with a Cr/Au metal electrode, and a graphene nanoribbon with a specific width was obtained by oxygen plasma etching using a hydrogen silicate resin as an etching mask. Two different types of graphene nanoribbon devices can be further prepared by this method (as shown in Fig. 4B and C), and the conductance of graphene nanoribbons with different widths and crystal orientations is measured (as shown in Fig. 4D ~ G), and it is found that the band gap value of the prepared graphene nanoribbons decreases with the increase of the width (as shown In Fig. 4h).
Bai et al. Reported a nanowire etching method to fabricate graphene nanoribbons with a width of less than 10 nm
[13]. The method comprises the following steps of: etching graphene attached to a silicon substrate by using oxygen plasma by using a silicon dioxide nanowire as an etching mask to obtain a graphene nanoribbon with a width similar to the diameter of the silicon nanowire. Due to the regularity of the nanowire mask used, the edges of the obtained graphene nanoribbons are relatively smooth, but the edge structure is still uncontrollable. Shin et al. Reported a method of using dip-pen nanolithography with polystyrene to prepare graphene nanoribbons that can be directly used for field effect transistor testing. The width of graphene nanoribbons depends on the diameter of the nanorods, which is generally less than 30 nm
[14].
2.2.2 Solution sonication
Dai et al. Reported a method to prepare graphene nanoribbons with high yield by sonication of expanded graphite in a dichloroethane solution of poly [ (m-phenylene vinylene) -co- (2,5-dioctyloxy-p-phenylene vinylene)]
[15].
According to the method, the width and the length of a polymer molecule in a solution are utilized to control the range of an interaction force between the polymer molecule and two-dimensional graphene, so that the breaking direction of the graphene is controlled to a certain extent, and the graphene nanoribbon with a narrow width distribution range is obtained. As shown in fig. 5, the width distribution of the graphene nanoribbons is 10 to 50 nm. The graphene nanoribbons prepared by the method show smooth edge structures, and the graphene nanoribbons applied to field effect transistors show high switching current ratio at room temperature.
2.2.3 Open Carbon Nanotube Method
Carbon nanotubes have small diameter and controllable number of layers, so graphene nanoribbons with controllable width and number of layers can be obtained after carbon nanotubes are opened along the length direction. Jiao et al. Successfully prepared graphene nanoribbons with smooth edges and narrow width distribution (10 ~ 20 nm) by argon plasma etching of multi-walled carbon nanotubes
[16]. As shown in fig. 6a, the upper surface of the multi-walled carbon nanotube is spin-coated with a polymethyl methacrylate film to protect part of the carbon nanotube, and graphene nanoribbons with different layers or even a single layer can be obtained by controlling the etching time of the film.
Kosynkin et al. Reported a method of oxidizing multi-walled carbon nanotubes in concentrated sulfuric acid solution to obtain graphene nanoribbons by longitudinal pyrolysis
[17]. As shown in Figure 6 B, carbon nanotubes may be cracked into graphene nanoribbons in an axial linear or helical manner, with the number of layers and width depending on the number of layers and diameter of the multi-walled carbon nanotubes used. The yield of graphene nanoribbons prepared by the method is close to 100%, but the conductivity of the graphene nanoribbons is reduced due to the introduction of more oxygen-containing functional groups on the surface of the graphene nanoribbon.
2.2.4 Organic synthesis
The above methods are all "top-down" methods, and the graphene nanoribbons prepared are wide and the edge structure is uncontrollable. The "bottom-up" method can overcome these two shortcomings and synthesize graphene nanoribbons with controllable width and edge structure.
The preparation of graphene nanoribbons by organic synthesis is mainly divided into two strategies: one is surface chemical synthesis on the substrate, and the precursor molecules are polymerized to form graphene nanoribbons; The other is to polymerize precursor molecules in solution to obtain graphene nanoribbons.
Cai et al. First reported the synthesis of armchair single-layer graphene nanoribbons of width N = 7 using surface chemistry
[18]. As shown in fig. 7A, the dibromodianthracene precursor molecules are thermally sublimated on the Au (111) surface and heated under high vacuum conditions, and the precursor molecules undergo a dehalogenation reaction and polymerize with each other to form a linear polymer chain; The temperature is continuously increased for annealing, and the graphene nanoribbon is obtained after cyclodehydrogenation. Langlais et al. Synthesized armchair graphene nanoribbons with a width of N = 3 on Ag (100) surface by photo-assisted surface synthesis method using 4,4 '-dibromo-p-terphenyl as precursor molecule, and found that photo-assisted surface synthesis could effectively reduce the reaction temperature
[19]. Surface synthesis controls the width and edge structure of graphene nanoribbons by designing precursor molecules. Therefore, graphene nanoribbons with various specific edge structures (such as zigzag) and atomically precise widths can be synthesized by using different precursor molecules and substrates
[20]. The disadvantage of this method is that the yield is low and it is difficult to achieve large-scale preparation. Transfer to other insulating substrates for application.
In order to overcome the above yield problem, different precursor molecules are designed to polymerize in solution, and graphene nanoribbons with different edge structures and precise widths can be prepared on a large scale
[22]. Narita et al. Reported the synthesis of structurally defined and easily processable armchair graphene nanoribbons in solution using specific precursor molecules
[23]. Vo et al. reported the preparation of armchair graphene nanoribbons with a width of nearly 1 nm and a length of more than 100 nm by polymer polymerization in solution (Fig. 7B)
[21].
2.2.5 Chemical vapor deposition
In the method, a metal substrate is used as a catalyst, so that hydrocarbons are decomposed at high temperature, C-H bonds are broken, and carbon atoms are self-assembled to form graphene nanoribbons
[24]. Kato et al. Used an electron beam etching process to etch and deposit nickel nanorods with a certain width on a SiO
2/Si substrate, and then heated the substrate and introduced a mixture of methane and hydrogen to obtain graphene nanoribbons with a width of about 23 nm on the surface of the nickel nanorods
[25]. Sakaguchi et al. Invented a radical polymerization chemical vapor deposition method to prepare armchair graphene nanoribbons with various widths in large quantities under low vacuum conditions
[26]. Chen et al. Prepared zigzag graphene nanoribbons by atmospheric pressure chemical vapor deposition and realized mass production
[27]. Fedotov et al. Reported the large-scale synthesis of high-quality armchair graphene nanoribbons of width N = 3 on nickel foil by low-pressure chemical vapor deposition
[28].
2.2.6 Template epitaxy
Sprinkle et al. used the principle of epitaxial growth of graphene on silicon carbide to print a nickel mask on the surface of silicon carbide and form a step by ion etching. Graphene nanoribbons with a width of about 40 nm could be grown on the step surface by heating at 1450 ℃. The width of graphene nanoribbons could be controlled by changing the etching depth to control the size of the step surface
[29]. Recently, graphene nanoribbons with controllable edge structure have been fabricated by a two-step method of crystal cutting and epitaxial growth
[30]. As shown in fig. 8, two-dimensional hexagonal boron nitride is catalytically cut by different kinds of metal nanoparticles to form nano-grooves with specific edges, and then graphene nanoribbons are epitaxially grown in the nano-grooves by chemical vapor deposition. Armchair and zigzag graphene nanoribbons with a width of less than 5 nm can be synthesized by this method. The edge structure of graphene nanoribbons depends on the type of catalytic nanoparticles. For example, zigzag graphene nanoribbons can be obtained by catalytic cutting with nickel nanoparticles, while armchair graphene nanoribbon can be obtained by catalytic cutting with platinum nanoparticles.
2.2.7 Carbon nanotube internal confinement synthesis method
In the method, a carbon nanotube is used as a nano reaction container, precursor molecules are filled into the carbon nanotube, and then the precursor molecules are decomposed and self-assembled and polymerized in the carbon nanotube under the action of heating or an electron beam, so that the graphene nanoribbon is formed.
Talyzin et al. Used polycyclic aromatic hydrocarbon hydrocarbons as precursor molecules to polymerize, cyclodehydrogenate and synthesize graphene nanoribbons with hydrogen atoms as terminal groups in single-walled carbon nanotubes
[31]. The width of the nanoribbon ranges from 0.5 to 1.0 nm, and its length is only limited by the length of the carbon nanotube, but its edge structure is not yet clear (Figure 9 a). Kuzmany et al. filled single-walled carbon nanotubes with an average diameter of 1.3 nm with ferrocene molecules as precursors, and the ferrocene molecules were heated and decomposed in vacuum and self-assembled into armchair graphene nanoribbons with N = 6 and 7 (Fig. 9b)
[32,33]. The method has the advantages that the edge structure and the width of the graphene nanoribbon can be accurately controlled by using different precursor molecules and carbon nanotubes with different diameters; The disadvantage is that the synthesized graphene nanoribbons are difficult to separate from carbon nanotubes. Chuvilin et al. Used sulfur-containing fullerene derivatives as precursor molecules in single-walled carbon nanotubes to synthesize graphene nanoribbons with sulfur atoms as terminal groups under electron beam irradiation
[34].
3 Graphyne nanoribbon
3.1 Structure and properties of graphyne nanobelts
Graphyne is a novel 2D carbon nanomaterial with hybrid of sp
1 and sp
2 (Fig. 10 a). For the first time, Li et al. Successfully synthesized a large-area graphyne film on the copper surface, and found that it has semiconductor properties and certain chemical stability, and the theoretical calculation showed that the stability of graphyne nanoribbons is higher than that of two-dimensional graphyne films
[35][36]. Unlike graphene nanoribbons, whose metal or semiconductor properties are determined by the edge structure, graphyne nanoribbons with zigzag and armchair edges are semiconductors, and their properties are not affected by the edge structure
[37]. Armchair graphyne nanoribbons have only non-magnetic States, while zigzag graphyne nanoribbons have either non-magnetic or ferromagnetic States
[38]. Bai et al. And Serafini et al. Calculated that the band gap of graphyne nanoribbons is an inverse function of the width of the nanoribbon, and the band gap decreases to a constant value with the increase of the width
[36,39]. The calculation results of Kang et al. Show that the band gap of graphyne nanoribbons can also be controlled by an external electric field, and the band gap decreases with the increase of the electric field strength, and the semiconductor-metal transition can be realized after the band gap decreases to the threshold
[40].
3.2 Preparation of graphyne nanobelts
Zhou et al. realized the coupling reaction of acetylene groups by chemical synthesis (as shown in Fig. 10B), and successfully synthesized graphyne nanoribbons with a width of 60 nm on highly oriented pyrolytic graphite, and measured that the prepared graphyne nanoribbons had an optical band gap of 1.91 eV
[41].
4 Biphenylene nanoribbon
4.1 Structure and properties of biphenylene nanobelts
Biphenylene is a novel graphene-like allotrope of two-dimensional carbon. Unlike graphene, it is composed of four-membered, six-membered and eight-membered rings of carbon. The theory predicts that the metal-semiconductor transition will occur when the biphenylene network is sheared into a nanoribbon structure
[42]. Denis used density functional theory to study the electronic structure of biphenylene sheets and nanoribbons, and found that the band gap of armchair biphenylene nanoribbons will decrease when the edge hydrogen is saturated
[43]. Fan et al. And Shen et al. Studied the electronic and optical properties of H-terminated biphenylene nanoribbons by theoretical calculation, and found that the band gap value of biphenylene nanoribbons decreased with the increase of width, and the direct-indirect band gap transition occurred
[44][45]. Karaush et al. And Ge et al. Theoretically predicted that zigzag biphenylene nanoribbons have the characteristics of semimetals, while armchair biphenylene nanoribbons have narrow band gap structures, and semiconductor-type biphenylene nano- ribbons have higher electron mobility than graphene nanoribbons
[46,47].
4.2 Preparation of biphenylene nanobelts
The main method for the preparation of biphenylene nanoribbons is surface chemical synthesis. Schluetter et al. Synthesized octa-substituted biphenylenes by chemical method, and then used Cu as an induced coupling medium to polymerize and cyclize the octa-substituted biphenylenes along different directions, and synthesized a variety of armchair biphenylene nanoribbons with different lengths and two different types of biphenylene networks
[48]. Fan et al. Reported the preparation of biphenylene nanoribbons using a two-step reaction
[44]. As shown in fig. 11A, dibromohexafluoroterphenyl is used as a reactant to deBr-couple on the Au (111) surface and polymerize to form poly (2,5-difluoro-p-phenylene) (PFPP chain); Then high temperature annealing is carried out to remove HF from the PFPP chain in an HF zipper mode, so that C-C coupling occurs between the chains, and finally the biphenylene nanoribbon is formed. H/F-terminated armchair biphenylene nanoribbons with different widths were synthesized as shown in Fig. 11 B.
5 Boron nitride nanobelt
5.1 Structure and properties of boron nitride nanobelts
The structure of BN nanoribbons is similar to that of graphene nanoribbons. However, unlike graphene nanoribbons, the band gap of armchair boron nitride nanoribbons oscillates with the width of the nanoribbon, while the band gap of zigzag boron nitride nanoribbons decreases monotonically with the width
[49]. Edge-fully hydrogenated armchair BN nanoribbons are non-magnetic semiconductors, while zigzag BN nanoribbons are magnetic and metallic and independent of the width of the nanoribbon
[50]. Chen et al. And Ding et al. Found that the semiconductor-semimetal-metal transition of partially hydrogenated zigzag boron nitride nanoribbons can be realized, and their conductivity and magnetism can be precisely controlled by controlling the hydrogenation ratio
[51,52]. Zhang et al. Found that the transition of boron nitride nanoribbons from insulator to metal can be realized by applying a transverse electric field
[49].
5.2 Preparation of boron nitride nanobelts
5.2.1 Open boron nitride nanotube method
Zeng et al. Obtained zigzag boron nitride nanoribbons with a width of about 15 nm by plasma etching multi-walled boron nitride nanotubes immobilized on polymethylmethacrylate
[53]. Erickson et al. obtained high-crystallinity few-layer boron nitride nanoribbons with uniform width (down to 20 nm) by intercalating potassium atoms into boron nitride nanotubes and longitudinally inducing the splitting of the nanotubes
[54]. Li et al. Mixed boron nitride powder with lithium oxide powder in a certain molar ratio and heated them in ammonia atmosphere to obtain a mixture of boron nitride nanotubes and boron nitride nanobelts, and the width of the multilayer nanobelts was about 26 nm, and the cracking rate was more than 40%
[55]. Boron nitride nanotubes were dispersed in aqueous ammonia solution and sonicated by Liao et al., and the boron nitride nanotubes were corroded and opened. Boron nitride nanoribbons with a width of 5 ~ 20 nm and a length of hundreds of nanometers could be prepared by this method
[56].
5.2.2 Solution sonication
Lin et al. Reported that boron nitride nanobelts were obtained using an ultrasonic hydrolysis method
[57]. Under ultrasonic conditions, the boron nitride bulk can be exfoliated into several layers or a single layer, and the B — N bond near the defect site is easily attacked by the oxygen ions in the water molecules, which further diffuses the defect, causing a part of the boron nitride sheet to shear to form a nanoribbon structure (Fig. 12A). Monolayer or few-layer BN nanoribbons with a width of less than 200 nm can be obtained by this method.
5.2.3 Graphene edge epitaxy
Han et al. Reported a "bottom-up" synthesis of boron nitride nanoribbons
[58]. As shown in fig. 12b, a single-layer boron nitride nanoribbon inheriting the graphene edge structure is epitaxially grown by using the graphene edge as a template through a chemical vapor deposition method, and the width of the single-layer boron nitride nanoribbon is related to the growth time and is up to hundreds of nanometers.
6 Phosphorene nanoribbon
6.1 Structure and properties of phosphorene nanoribbons.
Black phosphorus is a direct band gap semiconductor with a layered structure, and the layers are bonded by van der Waals forces. Few-layer black phosphorus is called phosphorene. In the structure of monolayer phosphorene, each phosphorus atom is connected with three surrounding phosphorus atoms through covalent bonds, forming a wrinkled honeycomb structure. When phosphorene is prepared into phosphorene nanoribbons, Vy et al. And de Sousa et al. Found through theoretical calculations that the band gap of phosphorene nanoribbons with different edge structures decreases with the increase of the width of the nanoribbon, whether hydrogenated or not, and when the width is infinitely close to phosphorene, the band gap decreases to 1.5 eV
[59,60]. And that conductivity of the armchair phosphorene nanoribbon can be Chan by applying an electric field, while the zigzag phosphorene nanoribbon is not affect by the applied electric field
[61,62]. Although phosphorene nanoribbons have many advantages, their chemical instability greatly limits the preparation and application of phosphorene nanoribbons due to their high chemical reactivity and susceptibility to oxidation and aging
[63].
6.2 Preparation of phosphorene nanoribbons.
6.2.1 Phosphene etching method
As shown in Fig. 13, Das et al. prepared few-layer phosphorene nanoribbons with a width of less than 10 nm by in-situ transmission electron microscopy etching
[64]. Phosphorene obtained by mechanical exfoliation is irradiated by electrons in a transmission electron microscope to make anisotropic pores with orientation dependence begin to expand, and then the edge control of armchair and zigzag nanoribbons is realized by a scanning transmission electron microscope with an accuracy of sub-nanometer level. Similarly, Feng et al. used electron beam lithography to fabricate armchair and zigzag phosphorene nanoribbons on black phosphorene with widths as low as 60 nm
[65].
6.2.2 Solution method
Watt et al. Prepared single-layer phosphorene nanoribbons with a width of 4 ~ 50 nm and an average length of about 690 nm by inserting lithium ions into black phosphorus crystals through a low-temperature amino method and dispersing the compound in an inert solution through mechanical stirring
[66]. Liu et al. Dispersed red phosphorus crystal powder in absolute ethanol and then carried out ultrasonic exfoliation to prepare oligolayer red phosphorus nanobelts on a large scale, with a thickness of less than 20 nm, a width of about 120 nm, and a length of several microns
[67].
7 Transition metal dichalcogenide nanoribbon
The transition metal dichalcogenide has a layered structure similar to graphite, and its chemical formula is MX2,M represents the transition metal elements, including Mo, W, Re, etc., and X represents the chalcogen elements S, Se, Te, etc. The following mainly introduces the properties and preparation of several transition metal dichalcogenide nanobelts.
7.1 Molybdenum disulfide nanobelt
7.1.1 Structure and properties of molybdenum disulfide nanobelts
The monolayer molybdenum disulfide is a hexagonal layered structure composed of covalent S-Mo-S trilayer atoms, and the planar chemical bonds between the layers are very strong, which ensures the stability of molybdenum disulfide
[68]. Monolayer MoS2 has a direct band gap, while multilayer MoS2 has an indirect band gap. The theoretical study shows that the ferromagnetism of zigzag MoS2 nanoribbons is metallic and independent of the width and thickness of the nanoribbons, showing ferromagnetism with magnetic moments mainly concentrated on the edge atoms, while armchair MoS2 nanoribbons are non-magnetic semiconductors
[69]. Fully hydrogenated armchair MoS2 nanoribbons can be tuned from indirect band gap to direct band gap semiconductors
[70]. Pan et al. Found that edge hydrogenation of armchair molybdenum disulfide nanoribbons leads to an increase in band gap, while for zigzag molybdenum disulfide nanoribbons, hydrogenation leads to an enhancement of magnetism
[71]. The band gap of armchair MoS2 nanoribbons oscillates with the increase of nanoribbon width, and the band gap finally saturates to 0. 56 eV with the increase of nanoribbon width, which is much lower than the band gap of monolayer MoS2 (1. 83 eV)
[69][72].
7.1.2 Preparation of molybdenum disulfide nanobelts
7.1.2.1 Open molybdenum disulfide nanotube method
Nanoribbons with specific width, edge structure and defects can be fabricated by laser cutting. Vasu et al. Prepared molybdenum disulfide nanobelts with a width of about 90 nm and a thickness of 20 ~ 30 layers by using pulsed laser to cut nanotubes longitudinally, and the thickness of the nanobelts could be adjusted by controlling the laser intensity
[73].
7.1.2.2 Carbon nanotube internal confinement growth method
Wang et al. Dispersed carbon nanotubes as a reaction vessel in a H
3PMo
12O
40 solution, and then heated the carbon nanotubes filled with H
3PMo
12O
40 at 800 ° C for 2 H under a H
2S/H
2 atmosphere to synthesize zigzag molybdenum disulfide nanobelts with a width of 1.5 – 4 nm
[74].
7.1.2.3 Chemical vapor deposition
Different from the previous vapor-solid-solid mechanism of growing two-dimensional materials, Li et al. Used the thermal evaporation method of vapor-liquid-solid mechanism to grow monolayer molybdenum disulfide nanoribbons on the substrate, with the width ranging from tens of nanometers to microns and the length ranging from several to tens of microns
[75]. Yang et al. Grew monolayer armchair MoS2 nanoribbons on SiO
2/Si covered with graphene, with widths ranging from 50 to 1000 nm and lengths ranging from 300 nm to 13 μm
[76]. Wu et al. Grown 1-3 layers of armchair molybdenum disulfide nanoribbons (Fig. 14) on a sapphire substrate, with a width of 0.2 ~ 1.4 μm and a length of about 10 μm
[77].
7.1.2.4 Stencil printing
Hung et al. Prepared dense arrays of molybdenum disulfide nanobelts using (NH
4)
2MoS
4 as ink by template printing combined with pyrolysis
[78]. During printing, a stamp with an engraved groove is placed on a substrate, (NH
4)
2MoS
4 solution is dripped on the edge of the stamp while pressure is applied on the stamp, and molybdenum disulfide nanobelts with the width of 157 to 465 nm can be obtained after the stamp is separated.
7.1.2.5 Electrochemical/chemical synthesis
Li et al. Synthesized molybdenum disulfide nanobelts by electrochemical, chemical synthesis two-step method
[79]. As shown in Fig. 15, the mixture of MoO
2 and MoO
3 was directionally deposited on the step edge of highly oriented pyrolytic graphite by electrochemical deposition, and then molybdenum disulfide nanobelts with a width of 50 – 800 nm were obtained by high temperature treatment in a H
2S environment, while molybdenum disulfide nanowires were obtained by low temperature treatment. The method can be used for obtaining the nanobelts which are directionally arranged according to the step direction, and the size of the MoO
x nanowire can be controlled by controlling the electric potential and time of the electrochemical deposition process, so that the width of the prepared molybdenum disulfide nanobelts can be adjusted to a certain extent. However, the width of the obtained nanoribbons is still wide, the edge structure is uncontrollable, and single-layer nanoribbons cannot be obtained.
7.2 Molybdenum diselenide nanobelt
7.2.1 Structure and properties of molybdenum diselenide nanobelts
Monolayer molybdenum diselenide has a sandwich "Se-Mo-Se" structure with a direct band gap of 1.5 eV
[80]. Zhang et al. Found that the edges of hydrogenated molybdenum diselenide nanoribbons can improve the stability of nanoribbons, and that fully hydrogenated or unhydrogenated armchair molybdenum diselenide nanoribbons are non-magnetic, while partially hydrogenated edges of nanoribbons are magnetic
[81]. Molybdenum diselenide nanobelts have good light absorption properties, so that the molybdenum diselenide nanobelts have application prospects in the field of solar cells
[82].
7.2.2 Preparation of molybdenum diselenide nanobelts
7.2.2.1 Liquid phase etching
Wang et al. Reported that molybdenum diselenide nanoribbons with a width of 300 ± 50 nm were obtained by etching molybdenum diselenide sheets using a reducing agent solution
[83].
7.2.2.2 Chemical vapor deposition
Chen et al. Used atmospheric pressure chemical vapor deposition to grow monolayer molybdenum diselenide nanobelts with a width of about 150 nm on monolayer molybdenum diselenide by placing the substrate on the upper end mixed with NaCl and MoO
3 powder and heating to a specific temperature in a Ar/H
2 atmosphere
[80].
7.2.2.3 MBE
Chen et al. Reported a "bottom-up" growth of molybdenum diselenide nanoribbons using molecular beam epitaxy
[84]. Zigzag molybdenum diselenide nanoribbons with a width of 10 ~ 30 nm were obtained by deposition of molybdenum and selenium vapors on highly oriented pyrolytic graphite. In particular, Poh et al. used molecular beam epitaxy (Fig. 16) to grow high-quality molybdenum diselenide nanoribbons with a width of about 15 nm on conductive highly oriented pyrolytic graphite and insulating hexagonal boron nitride substrates, and synthesized 1D-2D hybrid heterojunctions by two-stage growth method
[85].
7.2.2.4 Gold template assisted growth method
Cheng et al. Deposited selenium vapor on Au (100) surface and pre-assembled to form Se/Au template. Selenium and molybdenum vapors were simultaneously deposited on the Se/Au template, and the selenium and molybdenum self-assembled to form nanoribbons (fig. 17)
[86]. Because the nanoribbon is confined between two zigzag selenium chains, molybdenum diselenide nanoribbons with a width as low as 0.7 nm can be obtained.
7.3 Tungsten disulfide nanobelt
7.3.1 Properties of tungsten disulfide nanobelts.
Tungsten disulfide transforms from an indirect band gap to a direct band gap semiconductor when it changes from bulk to monolayer, and the electronic properties and magnetic properties of nanoribbons can be adjusted by applied stress
[87]. Zigzag tungsten disulfide nanoribbons are ferromagnetic metals, while armchair tungsten disulfide nanoribbons are non-magnetic semiconductors with strong optical polarization anisotropy
[88]. Han et al. Used first-principles calculations to find that tungsten disulfide nanoribbons with widths of N = 5 – 7 have high structural stability, and armchair tungsten disulfide nanoribbons with width of N = 5 have the largest carrier mobility
[89].
7.3.2 Preparation of tungsten disulfide nanobelts
7.3.2.1 Open tungsten disulfide nanotube method
Nethravathi et al. Added n-butyllithium to hexane to intercalate lithium atoms into tungsten disulfide nanotubes, and then reacted the lithiated nanotubes with long-chain thiols to cut the nanotubes longitudinally to produce nanoribbons with a width of about 100 nm
[90]. The disadvantage of this method is that the introduction of sulfur element into the system will functionalize the surface of the nanobelt, which makes it impossible to obtain pure tungsten disulfide nanobelt.
Nanobelts with specific width can be controllably prepared by laser cutting. Vasu et al. Prepared tungsten disulfide nanoribbons with an average width of 100 nm by pulsed laser longitudinal cutting of nanotubes, and the thickness of the nanoribbons could be controlled by controlling the laser intensity
[73].
7.3.2.2 Etch-displacement method
Heyne et al. Prepared tungsten disulfide nanoribbons on silicon substrates by pre-coating and printing
[91]. Photoetching a line with specific width on a silicon substrate, introducing WF
6 to react with silicon at high temperature, replacing the silicon with tungsten on the substrate, introducing H
2S gas to carry out in-situ sulfurization to generate tungsten disulfide,This method not only protects the nanobelt from plasma damage, but also accurately controls the thickness and width of the nanobelt by controlling the thickness and lithography width of the pre-deposited silicon. The width of the prepared nanoribbon is about 20 nm, and the thickness is about five layers.
7.3.2.3 Carbon nanotube internal confinement growth method
The method for preparing tungsten disulfide nanobelts grown in double-walled carbon nanotubes is similar to the method for preparing molybdenum disulfide nanobelts mentioned above, except that the solution is replaced by a H
3PW
12O
40, and then a single-layer tungsten disulfide nano-belt with a zigzag width of 1 to 3 nm is synthesized by using the confinement effect of the carbon nanotubes
[92].
7.4 Tungsten diselenide nanoribbon
7.4.1 Structure and properties of tungsten diselenide nanobelts
Tungsten diselenide is an inorganic compound with a layered structure, and single-layer tungsten diselenide is a typical sandwich structure composed of "Se-W-Se". Zigzag tungsten diselenide nanoribbons exhibit metallic behavior, while armchair tungsten diselenide nanoribbons are semiconducting
[93]. Shahbazi et al. Found that under the action of an applied transverse electric field, armchair tungsten diselenide nanoribbons undergo a semiconductor-metal phase transition, while zigzag tungsten diselenide nanoribbons exhibit an energy gap
[94]. Tungsten diselenide nanoribbons are considered to be a new type of high performance thermoelectric materials because the disordered edge caused by the dangling bonds at the edge of tungsten diselenide nanoribbons can hinder the phonon thermal conduction and improve the thermoelectric figure of merit
[93,95].
7.4.2 Synthesis of Tungsten Diselenide Nanobelt
7.4.2.1 Focused ion beam induced etching
Focused ion beam plays an increasingly important role in the field of nanotechnology because of its large ion mass, which can be used for etching, deposition, ion implantation and other micro-and nano-level processing of materials and devices. Stanford et al. Used focused helium ion beam induced etching to directly write tungsten diselenide nanoribbons with high resolution, high precision and width less than 10 nm by ion beam scanning pattern, but at the same time, it also produced many defects and damages to tungsten diselenide nanoribbons, and its edge structure and smoothness were difficult to control
[96].
7.4.2.2 Chemical vapor deposition
Duan et al. Prepared a variety of high-quality transition metal dichalcogenide nanoribbons under atmospheric pressure by using space-confined and substrate-guided chemical vapor deposition growth methods
[97]. As shown in Fig. 18, the selenium powder was placed upstream, and the WO
3 powder was dispersed between two pieces of SiO
2/Si substrates with a distance of 1 mm, heated and kept warm under the flow of H
2/Ar, and the tungsten diselenide nanobelt could be obtained on the substrate after cooling. The two stacked substrates provide a confined space and stable reaction conditions for the growth of nanoribbons, which makes the growth of nanoribbons change from thermodynamically stable two-dimensional to kinetically one-dimensional. The width of the monolayer tungsten diselenide nanoribbon synthesized by the method can be adjusted and controlled from tens of nanometers to several micrometers.
7.4.2.3 Epitaxial growth method
Aljarb et al. Reported a method to prepare tungsten diselenide nanoribbons with self-aligned single crystal structure by epitaxial growth, which can also be extended to the growth of other transition metal dichalcogenide nanoribbons and heterojunctions
[98]. Tungsten diselenide or molybdenum disulfide is preferentially oriented and nucleated on the step ridge formed on the (100) surface of the Ga
2O
3 substrate, and the tungsten diselenide arrays grown in small areas are finally mutually fused to form a continuous nanobelt, but the nanobelt obtained by the method has rough edges.
7.5 Tungsten ditelluride nanoribbon
7.5.1 Structure and properties of tungsten ditelluride nanobelts
Tungsten ditelluride nanoribbons have a similar sandwich structure to tungsten diselenide nanoribbons and inherit the excellent properties of tungsten ditelluride crystals, exhibiting topological properties such as Weyl half-metallicity and quantum spin Hall effect
[99]. Unlike tungsten ditelluride crystal, which has an indirect band gap, tungsten ditelluride nanoribbons have a direct band gap, and zigzag tungsten diteluride nanoribbons are metallic, while armchair tungsten diteluride nanorbbons are semiconducting
[100]. These excellent properties make tungsten ditelluride nanoribbons have great application prospects in the field of nanoelectronics, such as using the topological phase transition generated by the edge States of tungsten ditelluride nanoribbons as channel materials, which may realize the switching of topological field effect transistors
[101].
7.5.2 Preparation of tungsten ditelluride nanobelts
7.5.2.1 Chemical vapor deposition
Monolayer or few-layer tungsten ditelluride can be grown by chemical vapor deposition
[102,103]. On this basis, Li et al. Synthesized tungsten ditelluride nanobelts using a two-step reaction
[104]. The method comprises the following steps of: firstly, preparing a WO
3-x nanobelt by utilizing a hydrothermal method; and then, tellurizing the WO
3-x nanobelt by utilizing a chemical vapor deposition method under the N
2/H
2 of a heated reducing atmosphere to obtain the tungsten ditelluride nanobelt, wherein tellurium powder and the WO
3-x nanabelt are arranged separately. Tungsten ditelluride nanobelts synthesized by this method have a length of 3 ~ 5 μm, a width of 100 ~ 200 nm, and a thickness of 20 ~ 80 nm. The method has the advantages that the reaction conditions are mild, and the tungsten ditelluride nanobelt can be prepared on a large scale; However, the disadvantage is that the stoichiometric ratio of nanobelts is arbitrary and the impurity content is high.
7.5.2.2 Carbon nanotube internal confinement growth method
Kanda et al. Used WO
x molecules as precursors to fill WO
x molecules and tellurium vapor into carbon nanotubes, and under the condition of vacuum annealing, they reacted and grew tungsten ditelluride nanobelts in carbon nanotubes
[105].
7.5.2.3 Nanowire-nanobelt conversion method
Lim et al. Reported a nanowire-nanoribbon conversion method to prepare a variety of zigzag transition metal dichalcogenide nanoribbons in large area, including tungsten disulfide nanoribbons, tungsten diselenide nanoribbons and tungsten ditelluride nanorbbons
[106]. As shown in fig. 19A, one-dimensional tungsten telluride nanowires are used as precursor molecules and annealed in a chalcogen atmosphere to induce conversion into one-dimensional zigzag nanoribbons. The nanobelts prepared by the method grow in a vertical or horizontal geometry, depending on the thickness of the tungsten telluride nanowire.
7.6 Rhenium disulfide nanobelt
7.6.1 Structure and properties of rhenium disulfide nanobelts
Rhenium disulfide is a new type of two-dimensional VII-TMDs material. Rhenium disulfide has a S-Re-S sandwich structure. Because of its unique distorted 1T structure, rhenium disulfide shows unique properties different from other transition metal dichalcogenides. Due to the weak interlayer coupling, when rhenium disulfide is exfoliated from the bulk crystal (1. 5 eV) into a single layer (1. 6 eV), the band gap increases by only 0. 1 eV, and all of them show a direct band gap structure
[107]. After the preparation of nanobelts, it is still a semiconductor with a direct band gap, but due to the quantum size effect, the theoretical calculation predicts that the band gap of rhenium disulfide nanobelts gradually tends to 0. 92 eV with the increase of width
[108].
7.6.2 Preparation of rhenium disulfide nanobelts
7.6.2.1 Chemical vapor deposition
He et al. Synthesized few-layer rhenium disulfide nanobelts with a width of about 50 nm by introducing sulfur vapor into rhenium powder placed on a SiO
2/Si using a chemical vapor deposition method
[109].
7.6.2.2 Carbon nanotube internal confinement growth method
Norman et al. Used single-walled carbon nanotubes as a nano-reaction vessel, first filled Re
2(CO)
10 into carbon nanotubes, then filled iodine, and heated in a H
2S environment to form a single-layer rhenium disulfide nanobelt with a width of about 1.3 nm in carbon nanotubes
[110].
8 Conclusion and prospect
Theoretical and experimental results show that the properties of one-dimensional nanoribbons can be controlled by controlling the width and edge structure of the nanoribbons, resulting in more excellent properties and even new physical phenomena. For example, graphene with zero band gap can be prepared into armchair nanoribbons with a band gap of more than 2.3 eV. Zigzag graphene nanoribbons, on the other hand, have magnetic properties that they did not have before. As shown in Table 1, this paper summarizes several important one-dimensional nanoribbons of two-dimensional materials in recent years, and introduces their structures, properties, preparation methods and application prospects, with emphasis on the controllable preparation of nanoribbon width and edge structure and its impact on properties.
In the preparation of one-dimensional nanoribbons, the "top-down" and "bottom-up" methods are mainly introduced. Among them, the "top-down" two-dimensional material etching method can obtain nanoribbons with controlled width and edge, but the edge structure of nanoribbons prepared by this method is often irregular and uniform, and the width is more than 5 nm, the confinement effect is not obvious enough, so it is impossible to realize the preparation of complex edge structure. The nanotube opening method is also a "top-down" strategy, which can realize the preparation of nanoribbons. However, the opening process often requires extreme chemical conditions, resulting in poor quality of the resulting nanoribbons. Moreover, due to the disunity of the diameter/chirality of the nanotube itself, it is impossible to control the width, edge structure, and even the number of layers of the nanoribbon. Contrast this with the "bottom-up" synthesis strategy. Due to the abundance of various small organic molecules as monomers or raw materials, the width of nanoribbons can be precisely controlled, and the edge structure is not limited to armchair and zigzag. However, on the whole, the yield of the synthesized nanobelts is low, the length is limited, and there are many by-products. Specific to a certain method, each is also subject to some limitations, such as: surface chemistry is difficult to achieve large-scale preparation; The nanobelts synthesized by solution method and chemical vapor deposition method have many defects. It is also a difficult problem to take out the nanobelts synthesized in the confined area of carbon nanotubes.
To sum up, the existing methods still have the problems of difficulty in preparing ultra-narrow/ultra-long nanoribbons, specific orientation, specific edge structure control, defect control, large-scale production, and subsequent process integration. Therefore, how to precisely and controllably prepare high-quality and high-stability one-dimensional nanoribbons with specific width, length, thickness and edge structure on a large scale is still the focus and difficulty of future research. In addition, it will be a major challenge and innovation for future research to experimentally determine the new physical phenomena brought about by one-dimensional structures predicted by theory and apply the unique properties of these nanoribbons (which are not found in two-dimensional materials themselves). In particular, it should be pointed out that future research needs to design experiments according to the different requirements of applications and synthesize nanoribbons of specified types and functions. For example, the synthesis of single-layer nanoribbons with sub-nanometer width, more than micron length and directional arrangement in semiconductor applications; In the fields of catalysis and energy, large-scale synthesis of oligolayer nanoribbons with irregular edge structure and specific defects is needed. In infrared detection, terahertz emission, solar cells and other applications, it is necessary to control the synthesis of nanoribbons with specific energy gaps.
Guangzhou Basic and Applied Basic Research Foundation(202201011790)
National Natural Science Foundation of China(51902353)
Fundamental Research Funds for the Central Universities, and the Sun Yat-sen University(22lgqb03)
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