The present invention aims to provide a method of producing ultrathin crystalline layers of C8-BTBT, PTCDA and ultrathin layered heterostructures of them with precise control of thickness. The as-produced form of materials is applicable to various organic devices including light emitting diodes, light emitting transistors, thin film transistors, photodetectors, and organic quantum well superlattices and device applications therein.
Technique for producing ultrathin organic crystalline semiconductors and heterostructures with precise control is extremely important for material research and for realizing various organic devices including light emitting diodes, light emitting transistors, thin film transistors, photodetectors, and organic quantum well superlattices.
However, most techniques to produce organic thin films are difficult to precisely control the number of molecular layers. In commonly used thermal evaporation, the deposited films usually have variations in the local thickness[1]. Although self-assembled mono-layer (SAM) technique can be used to produce monolayer organic thin-film on many surfaces[2], it is challenging to realize layer-by-layer heterostructures for advanced electronic and optoelectronic device applications. Organic molecular beam epitaxy (OMBE) can achieve precise control on material's thickness and quality with the help of ultrahigh vacuum and in situ monitoring system[3], but it is difficult to achieve large-scale uniform layered crystal and the equipment needed is very expensive.
The above problem is due to the fact that organic crystals are bound by much weaker van der Waals (vdW) forces, rather than covalent bond in inorganic crystals which leads to easier control on atomical layers as molecular beam epitaxy (MBE) does[4]. The present invention is achieved by exploiting the vdW interactions and aiming to produce ultrathin organic crystalline semiconductors and heterostructures with much cheaper equipment.
The present inventors have conducted extensive studies and developed a method for precisely growing ultrathin mono- to few-layer crystal of C8-BTBT and PTCDA on a support, like graphene. In this method, source of the organic semiconductor material to grow and a support are spaced from each other in a vacuum chamber and subjected to a temperature gradient, and ultrathin organic semiconductor crystal can be deposited on support with precisely controlled molecular layers in a self-limited manner. The as-deposited ultrathin C8-BTBT or PTCDA crystal can be one-molecular-layer to few-molecular-layer in thickness and has full coverage on the support without any additional layers or defects. The thickness depends on the local temperature of the support, not on the deposition time. This aspect is highly desirable to reduce the variations brought by deposition time.
One aspect of the present invention relates to a method for precisely growing two-dimensional layers of crystal of an organic semiconductor material on a crystalline surface of a support. The organic semiconductor material can be C8-BTBT or PTCDA. The method comprises the steps of
In the tube furnace, an open container (about 1 cm in size) containing C8-BTBT powder (from Sigma-Aldrich co. LLC) was placed in the quartz tube chamber. Then the graphene sample was placed 2-10 cm away from the source. The quartz tube chamber was sealed and evacuated by a turbo molecular pump to about 4×106 Torr. The C8-BTBT powder was then heated to 120° C. to start the growth. After 5 growth, the furnace was turned off and the sample was cooled down to room temperature with the vacuum condition maintained. 5 minutes′, 10 minutes′, 20 minutes' and repeated growth was carried out. As a result, a monolayer of C8-BTBT was grown on graphene. These samples are shown in
In the same tube furnace, we replaced the source with PTCDA powder (from Sigma-Aldrich co. LLC) to perform the growth of PTCDA. The graphene sample was placed 2-5 cm away from the source. The quartz tube chamber was sealed and evacuated by a turbo molecular pump to about 4×106 Torr. The PTCDA powder was then heated to 280° C. to start the growth. 5 minutes′, 30 minutes' and repeated growth was carried out. As a result, a monolayer of PTCDA was grown on graphene.
The self-limited growth of C8-BTBT and PTCDA was also successfully repeated on hexagonal boron nitride (hBN) with other growth condition unchanged.
Another aspect of the present invention relates to a method for precisely growing heterojunction comprising two-dimensional layers of C8-BTBT and PTCDA on a crystalline surface of a support, and the method comprises the steps of
As a result, heterojunction of PTCDA monolayer and C8-BTBT bilayer was grown on graphene. This sample is shown in
The as-grown 2D organic crystal and organic heterojunction are applicable to various organic devices including light emitting diodes, light emitting transistors, thin film transistors, photodetectors, and organic quantum well superlattices.
The term “C8-BTBT” is short for 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene, a p-type small-molecule semiconductor (its formula is shown below).
The term “PTCDA” is short for perylene-3,4,9,10-tetracarboxylic dianhydride, an n-type small-molecule semiconductor (its formula is shown below).
The term “self-limited” herein is used to describe such a kind of growth which terminates itself after a specific layer forms completely, though given enough source and time. That means a “self-limited growth” can produce complete layered crystal without adlayers, which has hardly been achieved before in vdW epitaxial small organic crystal. That is why the method we invented was named “self-limited organic molecular beam epitaxy”.
Unless otherwise indicated, the term “two-dimensional (2D) layer” or “monolayer” used herein means a one-atom-thick or one-molecule-thick crystalline layer of a substance, but its thickness may vary because of different packing configurations of the molecules constituting the crystalline layer. For example, a monolayer of C8-BTBT is a one-molecule-thick layer of C8-BTBT, the thickness of which may be approximately 0.6 to 3 nm depending on the packing configuration of C8-BTBT molecules (see
The term “graphene” used herein refers to a monolayer of hexagonal carbon or a multiple layers of hexagonal carbon stacked upon one another. Graphene in the context of this specification may have a thickness of 0.3 to 10 nm, but not limited thereto.
The term “support” used herein refers to a physical base on which the organic semiconductor crystal can epitaxially grow. It supports epitaxy of organic crystal by providing a substantially smooth crystalline surface and van der Waals interaction, but is not necessarily rigid. For example, when the support is ultrathin graphene or hBN, it may be flexible.
The term “substrate” used herein refers to a physical base routinely used for an element or unit structure in electronic devices, which may comprise a metal, a metalloid, a semiconductor, an insulator, or a combination thereof. Substrate can also be flexible and optical transparent plastics. In the present invention, the support is positioned on the substrate in the specific examples disclosed. However, in other applications, the support may be the same as the substrate.
The term “vacuum” used herein refers to an environment at a pressure below one atmosphere (˜105 Pa, or 760 Torr).
Method for Precisely Growth of 2D Layered Crystal
In one aspect, the present invention relates to a method for precisely growing two-dimensional layers of crystal of an organic semiconductor material on a crystalline surface of a support. The organic semiconductor material can be C8-BTBT or PTCDA. The method comprises
The method of the present invention can achieve ultrathin 2D C8-BTBT and PTCDA crystal with full coverage on the support.
In an embodiment of the method, the vacuum chamber may be tube like and the support and the source of the organic semiconductor material are arranged horizontally in the tube-shaped chamber and are spaced from each other at a distance.
To achieve self-limited organic molecular beam epitaxy, the organic semiconductor material to be deposited should have a gradient of van der Waals forces near the interface of the support, by exploiting which we can achieve the precisely controlled growth. In a preferred embodiment, the organic material is C8-BTBT and the support is graphene. The C8-BTBT-on-graphene structure has been extensively investigated and the molecular packing near the interface was found to be different from bulk crystal of C8-BTBT[5]. The thickness of the neighbouring two layers (namely the interfacial layer, IL, and the first layer, 1L) is ˜0.7 nm and ˜1.7 nm, respectively (see
In a preferred embodiment of the method, the pressure in the vacuum chamber may be any value below 10 Torr, preferably 10−3 Torr or less, more preferably 10−5 Torr or less.
The support in the method according to the present invention is not specifically limited, and any material can be used as the support as long as it can provide a substantially atomically smooth crystalline surface and a gradient of van der Waals interactions near the interface. In a preferred embodiment, the support is graphene. In this case, any kinds of graphene can be used, for example, mechanically exfoliated graphene, CVD graphene, or epitaxial graphene. The thickness of graphene can be from monolayer to about 10 nm, but not limited thereto. In another preferred embodiment, the support is hBN.
In an embodiment of the method, the deposition time of the organic semiconductor is not an important parameter, as long as it is long enough for the layered organic crystal to form completely. In a specific embodiment, the organic semiconductor material is C8-BTBT and 5 minutes is enough for growth.
In a preferred embodiment, growth of C8-BTBT monolayer crystal on graphene can be achieved. The growth of the 2D C8-BTBT monolayer crystal by the method of the present invention can be confirmed by atomic force microscopy (AFM).
In another preferred embodiment, growth of C8-BTBT bilayer crystal on graphene can be achieved. Like
In a preferred embodiment, growth of PTCDA monolayer crystal on graphene can be achieved. PTCDA is a planar molecule favoring the face-on packing on graphene[7,8]. Although the structure, properties and evaporation temperature of PTCDA are very different from C8-BTBT, we are able to achieve growth of monolayer PTCDA on graphene.
Method for Precisely Growth of 2D Heterojunction
Another aspect of the present invention relates to a method for precisely growing heterojunction comprising two-dimensional layers of C8-BTBT and PTCDA on a crystalline surface of a support, and the method comprises the steps of
In a preferred embodiment, self-limited growth of the heterojunction of PTCDA monolayer and C8-BTBT bilayer on graphene can be achieved.
The area for the substrate or support uses in the present invention can be any size or any shape, between 50-500 um2.
In one embodiment, the area is between 50-100 um2. In another embodiment, the area is between 100-200 um2. In another embodiment, the area is between 200-300 um2. In another embodiment, the area is between 300-400 um2. In another embodiment, the area is between 400-500 um2.
Graphene was exfoliated on a 285-nm SiO2/Si substrate without further thermal treatment, to prepare a graphene sample having a surface area of about 50 μm2. The exfoliated graphene was characterized by optical microscope, AFM and Raman spectroscopy before growth to obtain its thickness and topology information. The growth was carried out in a tube furnace as shown in
C8-BTBT crystal was grown by the same method as in Example 1 except that the deposition time was changed to 10 minutes. As a result, a monolayer of C8-BTBT was grown on graphene. This sample is shown in
C8-BTBT crystal was grown by the same method as in Example 1 except that the deposition time was changed to 20 minutes. As a result, a monolayer of C8-BTBT was grown on graphene. This sample is shown in
C8-BTBT crystal was grown by the same method as in Example 1 except that the distance between the support and the source was changed to 12 cm. As a result, a bilayer of C8-BTBT was grown on graphene. This sample is shown in
C8-BTBT crystal was grown by the same method as in Example 4 except that the deposition time was changed to 10 minutes. As a result, a bilayer of C8-BTBT was grown on graphene. This sample is shown in
C8-BTBT crystal was grown by the same method as in Example 4 except that the deposition time was changed to 20 minutes. As a result, a bilayer of C8-BTBT was grown on graphene. This sample is shown in
C8-BTBT crystal was grown by the same method as in Example 4. As a result, a bilayer of C8-BTBT was grown on graphene. Then a repeated 5 minutes' growth was carried out on this sample, and the further repeated growth did not result in additional layers. This example is shown in
C8-BTBT crystal was grown by the same method as in Example 1 except that the support was changed to hBN. As a result, a monolayer of C8-BTBT was grown on hBN.
C8-BTBT crystal was grown by the same method as in Example 8 except that the deposition time was changed to 20 minutes. As a result, a monolayer of C8-BTBT was grown on hBN.
C8-BTBT crystal was grown by the same method as in Example 4 except that the support was changed to hBN. As a result, a bilayer of C8-BTBT was grown on hBN.
C8-BTBT crystal was grown by the same method as in Example 10 except that the deposition time was changed to 20 minutes. As a result, a bilayer of C8-BTBT was grown on hBN.
PTCDA crystal was grown by the same method as in Example 1 except that the source was replaced by PTCDA powder (from Sigma-Aldrich co. LLC), the heating temperature was changed to 280° C. and the distance between the support and the source was changed to 2 cm. As a result, a monolayer of PTCDA was grown on graphene. This sample is shown in
PTCDA crystal was grown by the same method as in Example 12 except that the deposition time was changed to 30 minutes. As a result, a monolayer of PTCDA was grown on graphene. This sample is shown in
PTCDA crystal was grown by the same method as in Example 12 except that the support was changed hBN. As a result, a monolayer of PTCDA was grown on hBN.
PTCDA crystal was grown by the same method as in Example 14 except that the deposition time was changed to 30 minutes. As a result, a monolayer of PTCDA was grown on hBN.
PTCDA crystal was grown by the same method as in Example 12. Then C8-BTBT crystal was grown by the same method as in Example 4 except that the substrate was replaced by the as-deposited PTCDA crystal on graphene. As a result, heterojunction of PTCDA monolayer and C8-BTBT bilayer was grown on graphene. This sample is shown in
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. For a person skilled in the art, the embodiments and examples disclosed herein may be varied or modified in many ways without departing from the scope of the disclosure and such variations and modifications are included in the scope defined by the appended claims.