Compound semiconductor nanocrystals (quantum dots) exhibit unique size-dependent optoelectronic properties making them attractive for a variety of applications, including ultrasensitive biological detection, high-density information storage, solar energy conversion, and photocatalysis. There is presently a great need for developing scalable techniques that allow efficient synthesis, size control, and functionalization of quantum dots, without a loss of the desirable optical properties. We report experimental results on the properties and surface modification of ZnSe nanoparticles grown by a continuous vapor-phase technique utilizing an axisymmetric counterflow jet reactor. Luminescent ZnSe nanocrystals were obtained at room temperature by reacting vapors of dimethylzinc:triethylamine adduct with hydrogen selenide, diluted in a hydrogen carrier gas. The two reactants were supplied from opposite inlets of the counterflow jet configuration and initiated particle nucleation in a region near the stagnation point of the laminar flow field. Surface modification of nanoparticles by adsorption of 1-pentanethiol was used to control the rate of particle coalescence. The counterflow jet technique can be scaled up for commercial production and is compatible with other vapor-phase processing techniques used in the microelectronics industry.