A precise electric field technique prints single quantum dots one at a time, enabling scalable arrays of quantum light sources for next generation photonic and quantum technologies.
(Nanowerk Spotlight) Placing a single quantum dot exactly where it should go sounds simple until the experiment begins. In liquid, the dots drift, cling to surfaces, and rarely land one at a time. Yet this precision is essential. Each dot is a semiconductor crystal that emits light as individual photons—packets of light that can carry information in quantum communication and computing. To work properly, those photons must couple precisely into microscopic channels that guide light across a chip. A single misplaced dot can cause a circuit to leak light instead of controlling it.
That demand for perfect alignment has revealed a major gap in manufacturing. Random deposition scatters particles unevenly. Mechanical placement reaches nanometer accuracy but only in slow, laboratory conditions. Printing can cover large areas quickly but cannot decide which particle goes where. Quantum photonic devices rely on single emitters, yet existing tools cannot combine accuracy with scale.
Advances in nanomaterial chemistry and electric-field-based printing have hinted at a way forward. Quantum dots can now be made with remarkable stability, and electrohydrodynamic printers that use electric fields rather than heat or pressure can already form features smaller than a micrometer. Even so, no method had managed to lift one nanoparticle from a liquid and place it exactly where it belonged without carrying along excess solvent or nearby particles.
A study in Advanced Materials (“Deterministic Printing of Single Quantum Dots”) by scientists at the University of Washington, demonstrates how to do it. The process, called single particle extraction electrohydrodynamics, or SPEED, uses a shaped electric field to pull a single quantum dot from suspension and deposit it on a surface with nanometer-scale precision. The printed dots remain intact and emit single photons even when coupled to tiny optical cavities, showing that quantum light sources can now be positioned and integrated under ordinary room conditions.
Overview of experiment. a) Reported droplet sizes using conventional mechanical inkjet and electrohydrodynamic (EHD) printing over the past 35 years, along with the results of this work. b) Schematic diagram of the EHD printing setup used here. Top inset: Fluorescence Light Microscopy (FLM) of EHD printhead. Bottom inset: Scanning electron microscopy (SEM) of a single EHD-printed quantum dot (QD). c) Graphic representation of CdSe QD cores (orange) with a tunable range of hexagonal diamond CdS shell (yellow) diameters. d, e) Bisected illustrations of EHD printheads dielectrophoretically overcoming (d) surface tension to print droplets of particle ensembles and (e) interfacial forces to print singular particles. (Image: Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The SPEED approach begins with a suspension of quantum dots in a mixture of octane and hexadecane. This solvent choice balances the particle’s motion against the liquid’s resistance. The dots themselves have cadmium selenide cores surrounded by thick cadmium sulfide shells. The shell enlarges the particle and increases its ability to polarize in an electric field, a property essential for extraction. Each dot measures about 70 nanometers across and is coated with organic molecules that keep it stable in the nonpolar liquid.
Traditional electrohydrodynamic printing pulls droplets of liquid from a nozzle. SPEED acts on the particle directly. A nonuniform electric field exerts a dielectrophoretic force, which moves the particle because it polarizes more strongly than the liquid around it. When that force becomes large enough, it draws the particle through the liquid surface and onto the target, overcoming surface tension that would otherwise hold it in place.
The researchers measured the surface tension of the solvent and the surface energy of the dots to calculate the energy barrier for escape. They compared this value with the predicted dielectrophoretic force, which depends on field strength, particle size, and polarizability. The model showed that when the quantum dot radius exceeds about 32 nanometers, the electric force is sufficient to pull it through the surface. That finding guided the choice of a thick shell.
In experiments, the team used a glass nozzle about five micrometers wide with an internal electrode. The silicon substrate was grounded. A voltage of roughly one thousand volts direct current, combined with an alternating signal up to one thousand volts at one kilohertz, created the extraction field. Each print attempt lasted about two seconds. Adjusting voltage and timing revealed a regime that consistently placed single particles.
Microscopy confirmed the precision. Under optimal conditions, around sixty percent of attempts produced exactly one quantum dot at the target. Some sites contained small clusters, while a few held nonemitting dots, but most showed a single particle and no residue. Because the force acts on the dot rather than the liquid, nozzle size does not limit accuracy. That feature makes the process compatible with multi-nozzle industrial printheads capable of parallel operation. Coordinated arrays of nozzles could eventually print large grids of emitters at high speed.
Optical tests showed that the printed dots remained efficient single-photon sources. Each emitted light near 624 nanometers with a spectral width of about 21 nanometers. The decisive test, the second-order correlation function, measures whether photons are emitted singly or in groups. Values below 0.5 indicate true single-photon behavior. The printed dots averaged about 0.33, confirming that the process preserved their quantum optical properties at room temperature.
To test integration, the researchers printed dots into nanophotonic structures. They fabricated a horseshoe-shaped cavity from silicon nitride on silicon dioxide with gratings that guided light outward. A dot printed at the center coupled its emission into the waveguide, and light emerged from the gratings. The cavity resonated near 614.5 nanometers with a quality factor of 12 500. Even within this structure, the dot maintained single-photon emission, proving that SPEED can produce functional components for quantum photonics.
Numerical modeling explained the mechanism. The simulations showed steep electric-field gradients near the liquid surface, strong enough to overcome surface tension. Because the cadmium sulfide shell has a higher dielectric constant than the solvent, the dot feels a strong pull toward regions of high field intensity. Smaller or thinner-shelled dots experience weaker forces and stay trapped, matching experimental observations. The extraction is nearly dry, minimizing contamination and leaving precise placement without solvent residue.
The same principles could apply to many other nanoparticles. Any particle stable in a nonpolar solvent and more polarizable than the surrounding liquid could, in theory, be positioned in the same way. The authors note that halide perovskites, metal oxides, and nanodiamonds might all be suitable if their surfaces and sizes are tuned appropriately. This flexibility points toward applications that go beyond light sources to nanoscale sensors and catalytic structures.
The technique also fits broader efforts to make manufacturing cleaner and more efficient. Additive processes generate far less waste than the etching and masking used in conventional lithography. SPEED works at room temperature and pressure, consumes minute quantities of material, and requires no vacuum or aggressive chemicals. Combined with existing multi-nozzle systems, it could enable arrays of quantum emitters built quickly and with minimal waste.
This study defines a clear framework for printing individual nanoparticles with precision. It quantifies the competing forces at play, identifies the materials that make the process effective, and verifies that the printed particles operate as reliable quantum light sources. By turning printing from droplet deposition to direct particle extraction, the work connects laboratory precision with scalable fabrication. It shows that individual quantum dots, once difficult to position reliably, can now be printed where they are needed, forming a practical foundation for future quantum photonic circuits.
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