Colloidal quantum dot molecules manifesting quantum coupling at room temperature

Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We utilize semiconductor nanocrystals as artificial atoms to form nanocrystal molecules that are structurally and electronically coupled. CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused via constrained oriented attachment. The possible nanocrystal facets in which such fusion takes place are analyzed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling and wave-function hybridization are manifested by a redshift of the band gap, in agreement with quantum mechanical simulations. Single nanoparticle spectroscopy unravels the attributes of coupled nanocrystal dimers related to the unique combination of quantum mechanical tunneling and energy transfer mechanisms. This sets the stage for nanocrystal chemistry to yield a diverse selection of coupled nanocrystal molecules constructed from controlled core/shell nanocrystal building blocks. These are of direct relevance for numerous applications in displays, sensing, biological tagging and emerging quantum technologies.

This process is repeated iteratively until the electron and hole energies converge. In most cases, three iterations are sufficient to obtain a convergence. The emission energy was calculated by: Where (1.76 eV) 3 is the energy gap of CdSe. is calculated in a way which avoids the consideration of the coulomb potential twice (both for the electron and both for the hole). where σ is the absorption cross section of the particle at the excitation wavelength (375 nm in our case), J and E Photon are the pulse energy and photon energy of the excitation light, respectively, and A is the spot area. The absorption cross-section was calculated according to the procedure reported elsewhere. 6 The value of the absorption cross-section for the 1.4/2.1 nm CQDs and the corresponding fused dimer at 375nm was found to be 2.3×10 -15 cm 2 and 5.7×10 -15 cm 2 , respectively.

Supplementary
Excitation power dependence study on single fused dimer: The flickering nature of the fluorescence in single fused dimers associated with a tri-exponential lifetime decay (comprising of ~2 ns, ~5 ns and ~25 ns components) was observed for the 1.4/2.1 Coupled CQDs. The fluorescence flickering nature was present upon changing the laser power and observed even at the lowest measurable excitation.
The intensity traces followed a single distribution of intensities (much narrower than the high excitation) without reaching an off state (Supplementary Figure 24). Upon excitation of the particle by a short pulse laser, a charge state of the exciton can be created along with the neutral exciton which can contribute to the appearance of short decay components 7,8 .  Fig. 5f in the main manuscript). This clearly indicates on the enhanced photo-charging effect in case of fused dimers. Associated with the flickering of the intensity, we observed a lifetime distribution through the intensity traces (lower intensity traces have shorter lifetime as shown in figure   5c). Upon the decrease in photo-charging not only the average lifetime increases (from 4.5 ns at <N>~0.18 to 7.2 ns at <N>~0.03), but also the lifetime distribution (commonly referred as lifetime flickering) narrows down.

Formation of coupled CQDs molecule dimers
We utilized silica nanoparticles as a template 9 for the fabrication of coupled CQD molecules. The detailed route-line is depicted in Supplementary Figure 1. This was performed by the following steps: 1. Fabrication of SiO 2 nanoparticles, coated by MPTMS. This kind of SiO 2 particle presents thiol groups on its outer surface, which are later used for the binding of the CQDs.
2. Core/shell CQDs binding to the SiO 2 particle surface: mixing a solution of the chosen core/shell CQDs with the SiO 2 nanoparticles allows their binding to the available thiol sites on the silica surface.
3. Growth of a secondary thin layer of SiO 2 on the QDs@ SiO 2 for masking: In this manner the CQDs are immobilized and cannot rotate or reorient while only a top hemisphere is remained exposed for further chemical functionalization of the CQDs.
4. Selective surface decoration of the CQDs by linker groups: Chemical grafting of a functional structure based on thiol group as linkers is then applied which selectively reacts only with the exposed NC hemisphere. Here, a tetrathiol ligand was added as a linker by a ligand exchange reaction on the exposed CQDs surface (for example oleyamine). Additional optional purification (size selective separation) of the dimers from free monomer and higher linked oligomers is possible in between steps 6 and 7, or after the fusion step. Step 1: SiO 2 nanoparticles synthesis.
Step 2: core/shell CQDs binding to the SiO 2 particle surface. Step 3: masking by a secondary thin layer of SiO 2 growth on the QDs@SiO 2 .
Step 4: selective surface decoration of the CQDs by linker groups.
Step 5: dimer geometry formation on the silica surface.
Step 7: fusion to form coupled CQD molecules.

CdSe@CdS Core/Shell NCs synthesis and characterization:
Supplementary Figure

Structural characterization
The core/shell structure of the monomer is directly identified by atomic resolution STEM-HAADF curve -integrated intensity of SAED (k)).

Step I -Silica nanoparticles characterization
The SiO 2 nanoparticles (step 1) were prepared as described in the methods section 11  Step

II -Characterization of the Silica-QDs conjugates
The SiO 2 @QDs particles were prepared in step 2 by adding the CdSe/CdS CQDs to the SiO 2 nanoparticles solution. The resulting particles were characterized by TEM and SEM as shown in Supplementary Figure 5. In order to avoid the CQDs overlap and aggregation on the SiO 2 surface, the ratio of CQDs added to the SiO 2 nanoparticles was controlled. In the sample below, a 1:500 SiO 2 :QD ratio yielded well-separated and clearly resolved surface distribution of CQDs. The CdSe/CdS@SiO 2 nanoparticles solution was cleaned twice from free and weakly bound CQDs by centrifugation, discarding the supernatant and re-dispersion in toluene. Step IIIformation of a silica masking layer

Supplementary
The secondary masking silica layer provides two functions: firstly, coverage of the inherent surface thiol groups of MPTMS in order to avoid the adsorption of additional CdSe-CdS CQDs in the dimerization of step 5. 11 This enhances the efficiency of the dimer formation versus monomers binding.
Secondly, this immobilizes the CdSe-CdS CQDs such that they cannot rotate, exposing a hemisphere which emerges in the solvent and can be modified selectively by the chemical grafting of a functional structure/group.
In order to control the thickness of the secondary SiO 2 masking layer, an optimized mixture of PVP and TEOS was necessary. If the PVP amount is too low, it usually results in inefficient masking.
Conversely, if it is too high it could lead to a full QDs masking by the SiO 2 layer, which prevents the dimer formation in the next step. Additionally, the optimization of the TEOS amount was performed as described below. According to the calculation, 50 L of TEOS leads to a hemisphere mask. Three Step

IVlinker binding
For the dimer formation step, the chosen linkers bind to the exposed region of the anchored CQDs (step 4). A Tetra-thiol molecule was used as a bi-dentate linker molecule (Supplementary Figure 7).
The thiol groups strongly bind to the CQD surface and can displace the existing surface ligands of the exposed CQD hemisphere. In order to enhance the conjugation of the linkers, the surface modification procedure was performed under Argon flow, at 60 °C overnight. The excess linker molecules were removed by precipitation and centrifugation. Here, the cleaning step after the linker addition was significant for achieving high dimer formation yield. Figure 7. Chemical structure of the linker (pentaerythritol tetrakis(3-mercaptopropionate)).

Supplementary
Step V -Dimer formation Step

VI -Dimers release
Precisely controlled dimer CdSe-CdS CQD molecules were successfully achieved as follows. The release and separation of the CdSe-CdS CQDs dimers from the SiO 2 spheres was performed by selective etching process of the SiO 2 using an HF/NMF (10%) etching solution. The free dimers are shown in Fig. 1c. The freed dimers were separated by centrifugation decanting the supernatant and repeated ethanol precipitation/centrifugation cycles for three times. Step

VII -Fusion to form the coupled NC molecule
The fusion procedure plays a significant role in reduction of the potential barrier in the coupled CQDs molecules. The choice of correct temperature, precursor amount and ligands lead to the fusion of two CQDs without ripening and collapse.

Size selective separation process for 1.9/4.0 nm CQDs dimers
The fused dimer molecules as prepared by the procedure explained in the previous section contain some unreacted monomer CQDs which have been purified to achieve high yield of CQD molecule for further studies. Supplementary Figure 9 shows the released fused dimers, which were separated by

Absorption spectrum measurement for 1.4/2.1 nm CQDs
The features in the absorption spectrum change significantly after the fusion of two CQDs. To further demonstrate the changes, we have normalized the data at the bulk absorption regime (300nm). It can be observed that the band-edge absorption feature of the unfused dimer is retained and similar to the monomers. Upon fusion broadening is seen, along with a red shift and lower absorption. The excited state features are also broadened significantly upon fusion. Upon normalization at the band-edge, the significant relative change in absorption at higher energies upon fusion is emphasized (Supplementary Figure 11a). All these aspects indicate coupling effects upon fusion and formation of the CQD molecules.

Control experiment: monomer particles that were treated with the fusion procedure
The CQDs are exposed to different conditions during the synthetic procedure such as binding, etching, fusion etc. An important control is to identify whether the inherent properties of the CQDs change during these processes, especially during etching and fusion. Hence, we studied the absorption and photoluminescence spectra (Supplementary Figure 12)  20

Spectral characterization of the various fused CQDs dimers
The red shift of the fluorescence was enhanced with a size decrease in the CdSe-CdS CQDs composing the fused dimers. That is, the smaller core-shell CQDs molecules present strong coupling properties. Additionally, upon fusion, the full width at half maximum (FWHM) measured from the fluorescence spectrum was obviously increased compared with that of the monomer sample. For the atomic model, the Cadmium atoms are marked in brown and Sulfur atoms in blue.

Structural characterization of the fused 1.4/2.1 nm CQDs dimers
In our system, the attachment and fusion of the monomer CdSe/CdS CQDs was based on the linker's binding. Therefore, the attachment orientation relationships for the smaller CdSe/CdS CQDs molecules were similar to those observed for the larger CQDs. Yet, a small difference was still noticed.

/2.1 nm CQDs and the corresponding dimers
The fluorescence decay of a single 1.4/2.1 nm CQD follows mostly a mono-exponential decay of ~30ns or a bi-exponential decay with a small contribution from a 5 ns component. The average lifetime for the single dimer particles is clearly quenched and follows mostly a tri-exponential decay.
The distribution of the average lifetime for dimer particles is shifted to short values, which is more pronounced for the fused dimers than for the unfused ones.

Comparison of g 2 value for monomer and fused dimer
The antibunching at zero time delay accounts for the ratio of biexciton to exciton quantum yield (QY)

Attributes of fluorescence from single unfused dimer
The fluorescence from single unfused dimers also exhibited flickering nature instead of distinct on-off characteristics and followed a multi-exponential decay. The lifetime is not uniform throughout the intensity range, but rather presents distributions when analyzed at different intensity levels (Supplementary Figure 23(ii)). The single unfused dimer also gives rise to lower antibunching contrast. (b) Intensity distribution of the traces in (a). Narrow distribution is observed with decreasing laser power.