Figure 1

High-pressure reflection high-energy electron diffraction (HP-RHEED) patterns of NTCDA/DB-TCNQ growth on single-crystal KBr by organic vapor phase deposition. (a) HP-RHEED pattern for the KBr substrate scaled by a factor of 0.75$x$. HP-RHEED patterns of the first (b), (c), second (d), (e), and third (f), (e) pair growth of NTCDA (5 nm)/DB-TCNQ (5 nm). Congruent growth of DB-TCNQ layers (c), (e), (g) are grown at $T_{\mathrm{sub}}=0$ °C and $r_{\mathrm{dep}}=0.4$ nm/s on proceeding layers of NTCDA (b), (d), (f) grown at $T_{\mathrm{sub}}=25$ °C and $r_{\mathrm{dep}}=0.15$ nm/s. Positions of the diffraction streaks are highlighted by dashed lines. Note that the central streak (c), (e), (g) separates into multiple streaks indicating surface roughening with increasing number of layers. The electron-beam energy and current were 20.0 keV and < 0.1 μA, respectively. (right) Schematic structural model of the multilayer NTCDA (red)/DB-TCNQ (blue, yellow) crystal structure with the individual molecular structures shown in (h); K: maroon; Br: purple; C: gray; O: red; N: blue; S: yellow; and H: white.Reuse & Permissions

Figure 2

Rotation dependence of RHEED patterns for the first layer of NTCDA (a)–(c), and the second layer (i.e., 2nd pair) of DB-TCNQ (d)–(f) for the growth in Fig. 1. The measured $d$ spacings for NTCDA are (a) (10), (20), (30)$=$0.491, 0.332, 0.250 nm, respectively, (b) (02), (04)$=$0.652, 0.331 nm, respectively, and (c) (12), (13), (22), (24)$=$0.492, 0.393, 0.240, 0.203 nm, respectively. The measured $d$ spacings for DB-TCNQ are (d) (10), (30)$=$0.849, 0.272 nm, respectively, (e) (01), (03)$=$0.984, 0.323 nm, respectively, and (f) (11)$=$0.805 nm. Note that diffraction stemming from the first-order Laue zone in (a) can be observed for NTCDA. The NTCDA alignments are $[10]{}_{\mathrm{N}}//[100]{}_{\mathrm{KBr}},[01]{}_{\mathrm{N}}//[010]{}_{\mathrm{KBr}}$, and $[12]{}_{\mathrm{N}}~//[110]{}_{\mathrm{KBr}}$ and the DB-TCNQ alignments are $[10]{}_{\mathrm{D}}~//[130]{}_{\mathrm{KBr}}$, $[01]{}_{\mathrm{D}}//[010]{}_{\mathrm{KBr}},[11]{}_{\mathrm{D}}~//[320]{}_{\mathrm{KBr}}$.Reuse & Permissions

Figure 3

X-ray-diffraction patterns for single and multilayers of NTCDA and DB-TCNQ. The diffraction peaks in the multilayer structure are a simple convolution of the (100) and (001) peaks seen in the single-layer diffraction for NTCDA and DB-TCNQ. The normal direction alignments of these two lattices are therefore $(100){}_{\mathrm{N}}//(001){}_{\mathrm{D}}$. Note that multiple diffraction orders ($n$00) and (00$n$) are observed for NTCDA and DB-TCNQ, respectively, and the KBr (002) peak is seen at 2θ$=$27.80°.Reuse & Permissions

Figure 4

Model of the real-space overlayer alignment (a), (b) for DB-TCNQ (b, left) and NTCDA (b, right) on KBr diagrammed with (b) and without (a) the molecules in the unit cell; drawings are to scale. In (b), the molecular alignment within the unit cell is assumed from the bulk phase crystal structure. The potassium ions are slightly smaller than the bromine ions, and the KBr unit cell is indicated. The reciprocal-lattice vectors ($b^{*}$) are also highlighted for NTCDA and DB-TCNQ.Reuse & Permissions

Figure 5

Transmission electron microscope (TEM) diffraction pattern (a) from an NTCDA/DB-TCNQ bilayer transferred from the KBr substrate via aqueous solution etching. The electron beam is oriented normal to the bilayer surface and $(001){}_{\mathrm{KBr}}//(100){}_{\mathrm{N}}//(001){}_{\mathrm{D}}$. (b) TEM pattern from (a) overlaid with the measured reciprocal-lattice map. Note that slight smearing of the diffraction spots likely stems from a combination of growth-related and wet-transfer-induced dislocations. This map is consistent with the picture obtained from HP-RHEED, except that two rotations of NTCDA are observed: one of much lower diffraction intensity and rotated by 90° to the other (this rotation is not observed in HP-RHEED). The alignment of the $[01]{}_{\mathrm{D}}/[01]{}_{\mathrm{N}}$ [i.e., $b_1(\mathrm{D})//b_1(N)$] and $(001){}_{\mathrm{D}}//(100){}_{\mathrm{N}}$ are also consistent with Fig. 3. Note that the diffraction spots yield the $d$ spacing of the surface mesh since the monoclinic/triclinic (hkl) reciprocal-lattice points lie slightly out of plane (also leading to a relatively low diffraction intensity). Diffraction data were obtained at a beam energy of 300 keV.Reuse & Permissions

Figure 6

van der Waals potential energy (left axis) and geometric lattice potential ($V/V_0$) (right axis) as a function of adlayer-substrate azimuthal angle (θ) for (a) NTCDA on KBr, (b) DB-TCNQ on KBr, and (c) DB-TCNQ on NTCDA for adlayer surface mesh sizes comprised of $3\times 3$ unit cells (orange line) and $15\times 15$ unit cells (black line). (b) (Inset) The rotation angle (θ) was defined as the angle between $b_1$ of the adlayer ($b_1^A$) and substrate ($b_1^S$) defined in Fig. 4. Surface lattice parameters for NTCDA and DB-TCNQ were those measured from RHEED data and $a_1$$=$$a_2$$=$0.66 nm for KBr. A 6--12 Lennard-Jones potential was used to calculate the PE and only interactions between the substrate and adlayer were included in the summation (surface energies incorporating the summation of intralayer potentials are provided in Table ). Note that 0° and 90° are symmetrically equivalent for NTCDA and DB-TCNQ on KBr, but not for DB-TCNQ on NTCDA. Measured rotation angles are highlighted with blue arrows. For the dimensionless geometric potential, a value of $V/V_0=1.0,0.5$, and 0.0 indicates an incommensurate, coincident, and commensurate layer, respectively. Note that simple geometrical lattice considerations do not always elucidate energetic minima necessary to describe quasiepitaxy.Reuse & Permissions