Selective Laser-Assisted Synthesis of Tubular van der Waals Heterostructures of Single-Layered PbI₂ within Carbon Nanotubes Exhibiting Carrier Photogeneration

Abstract

The electronic and optical properties of two-dimensional layered materials allow the miniaturization of nanoelectronic and optoelectronic devices in a competitive manner. Even larger opportunities arise when two or more layers of different materials are combined. Here, we report on an ultrafast energy efficient strategy, using laser irradiation, which allows bulk synthesis of crystalline single-layered lead iodide in the cavities of carbon nanotubes by forming cylindrical van der Waals heterostructures. In contrast to the filling of van der Waals solids into carbon nanotubes by conventional thermal annealing, which favors the formation of inorganic nanowires, the present strategy is highly selective toward the growth of monolayers forming lead iodide nanotubes. The irradiated bulk material bearing the nanotubes reveals a decrease of the resistivity as well as a significant increase in the current flow upon illumination. Both effects are attributed to the presence of single-walled lead iodide nanotubes in the cavities of carbon nanotubes, which dominate the properties of the whole matrix. The present study brings in a simple, ultrafast and energy efficient strategy for the tailored synthesis of rolled-up single-layers of lead iodide (i.e., single-walled PbI nanotubes), which we believe could be expanded to other two-dimensional (2D) van der Waals solids. In fact, initial tests with ZnI already reveal the formation of single-walled ZnI nanotubes, thus proving the versatility of the approach.

An overview of the recent advances in inorganic nanotubes

“…7 (a-b)). 68,[70][71][72] This solvent-free high temperature approach was based on the use of the inner cavities of CNTs as hosting templates. Thus, the growth of lead diiodide single wall nanotubes (SWNTs), with diameters ranging from 3.5 to 7.5 nm, as well as nanowires, were reported, see Fig.…”

“…70 In a related work this group has laser ablated PbI 2 (and ZnI 2 ) in the presence of CNT mesh, obtaining core-shell single wall PbI 2 @CNT nanostructures in high yields. 72 Similarly, 73 a complex series of core-shell nanostructures were obtained by annealing layered GdI 3 in the presence of CNT mesh. In the case of double wall CNT with open end and hollow core of only 0.3 nm, pure iodine nanowires were encapsulated.…”

Structure of inorganic nanocrystals confined within carbon nanotubes

“…6(a) shows a HRTEM image of a PbI 2 -NT@MWCNTs as prepared by high temperature filling. A large variety of materials have been shown to also coat the inner cavities of MWCNT forming single-layered inorganic nanotubes (SLINTs), namely PbI 2 [57,58], ZnI 2 [58,59], CeCl 3 [59], CeI 3 [59], TbCl 3 [59], GdCl 3 [29], SmCl 3 [29], BiI 3 [60], GdI 3 [61] and BiCl 3 [62]. The role that the temperature of treatment has on the grown nanostructures was investigated using ZnI 2 and MWCNTs [59].…”

“…6(b, c)). Not only the crystal structure, but also the optoelectronic properties of the material were found to change drastically upon encapsulation [58].…”

Morphological Phase Diagram of Gadolinium Iodide Encapsulated in Carbon Nanotubes

“…[11][12][13] Apart from CNTs various inorganic materials such as chalcogenides, halides, oxides and metals can be synthesized in a form of nanotube and can also be encapsulated [14][15][16][17][18] . There are various methods to encapsulate substances inside CNTs and these have been reviewed in the past [19][20][21][22][23][24] . One of the most popular is the use of melts which relies on surface tension and capillary forces to drive inorganic compounds into the interior of the nanotube 2,25 .…”